SDL2: hardware-accelerated gfx_display + gfx_widgets backend

Brings the SDL2 video driver to feature parity with the modern menu
drivers (XMB / Ozone / MaterialUI / RGUI) plus widgets, FPS overlay,
Display Statistics, input overlay, and load-content animation.

Previously the SDL2 driver only supported RGUI's software bitmap
path: gfx_display dispatch was wired to no driver, get_overlay_interface
was NULL, set_osd_msg unconditionally went through the legacy bitmap
font, and the video size was never published to video_driver_set_size.
Anything beyond RGUI either silently no-op'd or rendered to the wrong
place against the wrong dimensions.

This adds:

  - gfx_display_ctx_sdl2:
      * blend_begin / blend_end (SDL_RenderSetDrawBlendMode)
      * draw - two paths: a plain-quad path for gfx_display_draw_quad
        callers (vtx == NULL, n == 4) and a vertex-array path for
        slice / 9-patch / per-vertex-color callers.  Both honour the
        canonical gl1_menu_vertexes triangle-strip layout (vertex 0
        = bottom-left in GL bottom-up coords -> bottom-left in SDL
        screen space after Y flip), the top-down tex_coord
        convention from gfx_display.c, draw->scale_factor for XMB
        tab zoom, and draw->rotation for the spinning hourglass
        (rotate corners around centre using cosf/sinf).
      * scissor_begin / scissor_end (SDL_RenderSetClipRect, with the
        bottom-up to top-down flip RA's scissor convention requires).
      * pipeline draws (ribbon / snow / bokeh) are stubbed - SDL's
        high-level renderer doesn't expose custom shaders, so these
        return early and XMB falls back to the static gradient.

  - sdl2_raster_font:
      * FreeType atlas uploaded as ARGB8888 with white-RGB and the
        FreeType alpha (so SDL_SetTextureColorMod tints crisply).
      * render_msg with newline splitting via a render_message
        wrapper that walks '\n' and offsets each segment by one
        line-height (XMB sublabels and Display Statistics depend on
        this).
      * get_glyph / get_message_width / get_line_metrics for upstream
        consumers (font_flush no-ops; bind_block is unused, every
        call goes through immediate-draw).

  - poke->set_osd_msg dispatch:
      * Was the bug masking proper menu/widget text - the original
        driver hardcoded the bitmap OSD font.  Now dispatches to the
        supplied font's render_msg when a font_data_t* is passed,
        falling back to the bitmap font only for the legacy
        font=NULL OSD path.

  - poke->load_texture / unload_texture:
      * Was NULL.  Without it, gfx_white_texture is never registered
        and every plain-quad call has no source texture, so widgets
        and menu items rendered as solid-colour blocks with no
        alpha modulation.

  - get_overlay_interface (input overlay):
      * Six callbacks (enable, load, tex_geom, vertex_geom,
        full_screen, set_alpha) plus a per-frame render path.
      * Coordinate convention: vertex_geom stores values verbatim
        (no y-flip).  SDL_RenderCopy operates in y-down pixel space
        like GDI's AlphaBlend, so the d3d/gl-style flip would be a
        bug here - mirrors gdi_overlay_vertex_geom.

  - Display Statistics overlay:
      * font_driver_init_osd / free_osd to register the SDL2 raster
        font as video_font_driver.  Renders stat_text via
        font_driver_render_msg with osd_stat_params, suppressed
        while the menu is alive (matching gdi/d3d8/d3d9).

  - Frame-loop wiring:
      * menu_driver_frame, gfx_widgets_frame, the stats overlay,
        and the input overlay all need the full-window viewport
        rather than the aspect-corrected game viewport.  Wrapped
        each in SDL_RenderGetViewport / SetViewport / restore so
        the next frame's core blit lands in the right place.

  - video_driver_set_size in sdl_refresh_viewport:
      * Tells the rest of RA the actual window size.  Without it
        video_st->width/height retain core geometry (e.g. 320x240),
        widgets size themselves to that tiny coordinate space, and
        the entire UI ends up drawing into the top-left corner of
        the framebuffer.  Mirrors what vga / gx2 / d3d8 / d3d9
        common do.

  - Settings whitelist (configuration.c:check_menu_driver_compatibility):
      * Added 's' case so XMB / Ozone / MaterialUI no longer get
        rejected when the active video driver is sdl2.

  - sdl2_raster_font registered in:
      * gfx/font_driver.c (FONT_DRIVER_RENDER_SDL2 dispatch)
      * gfx/font_driver.h (extern declaration)
      * gfx/video_defines.h (FONT_DRIVER_RENDER_SDL2 enum)
      * gfx/gfx_display.c / .h (GFX_VIDEO_DRIVER_SDL2 enum +
        gfx_display_ctx_sdl2 entry in the drivers table)

Requires SDL >= 2.0.18 for SDL_RenderGeometry; older SDL keeps the
RGUI-only behaviour via #if SDL_VERSION_ATLEAST guards.

Tested at 4K@120Hz fullscreen, plus all four menu drivers, the
neo-retropad touch overlay, FPS / Display Statistics overlays, and
the load-content animation against d3d9_hlsl as the reference
backend.  Visual output matches d3d9_hlsl pixel-for-pixel except for
the deliberately-stubbed XMB pipeline shaders.

Author: LibretroAdmin

configuration.c

@@ -3599,6 +3599,15 @@ static bool check_menu_driver_compatibility(settings_t *settings)
          return (memcmp(video_driver, "metal",  5) == 0 && video_driver[5] == '\0');
       case 'r':
          return (memcmp(video_driver, "rsx",    3) == 0 && video_driver[3] == '\0');
+      case 's':
+         /* sdl2 supports the full menu set (XMB/Ozone/MaterialUI/RGUI)
+          * via gfx_display_ctx_sdl2 + sdl2_raster_font, gated on
+          * SDL_RenderGeometry (>= 2.0.18). On older SDL builds the
+          * driver self-disables the gfx_display backend, and only
+          * RGUI's bitmap path is functional - which already returned
+          * true via the rgui early-out above, so allowing sdl2 here
+          * is safe regardless of the runtime SDL version. */
+         return (memcmp(video_driver, "sdl2",   4) == 0 && video_driver[4] == '\0');
       case 'c':
          return (memcmp(video_driver, "ctr",    3) == 0 && video_driver[3] == '\0');
       default:

gfx/common/sdl2_common.h

@@ -64,6 +64,31 @@ typedef struct _sdl2_video
    uint8_t font_b;
 
    uint8_t flags;
+
+#ifdef HAVE_OVERLAY
+   /* On-screen input overlay state.  Each entry holds an SDL_Texture
+    * uploaded as ARGB8888 (matching the supports_rgba=false byte
+    * layout RetroArch hands us in load()) and rendered via
+    * SDL_RenderCopy with per-texture alpha modulation each frame.
+    *
+    * vert_coords / tex_coords are 4-float (x, y, w, h) tuples in
+    * 0..1 normalised space.  vertex_geom flips y to (1.0f - y) and
+    * negates h, matching d3d8 / gl / d3d9_common; vertex_geom
+    * comments document the rationale - we undo the flip at render
+    * time in sdl2_overlays_render. */
+   struct sdl2_overlay
+   {
+      SDL_Texture *tex;
+      unsigned  tex_w;
+      unsigned  tex_h;
+      float     tex_coords[4];
+      float     vert_coords[4];
+      float     alpha_mod;
+      bool      fullscreen;
+   } *overlays;
+   unsigned overlays_size;
+   bool overlays_enabled;
+#endif
 } sdl2_video_t;
 
 void sdl2_set_handles(void *data, enum rarch_display_type