meta-stage: Remove backend assertions from overlay paint/update paths

Software cursor overlays are required as a fallback when the cursor
theme is missing a required cursor name.

Two assertions were preventing X11 from using this fallback, resulting
in a crash.

See the bug report below for details.

Fixes #801

With this commit in place, we no longer crash. We get a missing cursor
instead.

Author: clefebvre

src/backends/meta-stage.c

@@ -126,8 +126,6 @@ meta_overlay_paint (MetaOverlay         *overlay,
   if (!overlay->enabled)
     return;
 
-  g_assert (meta_is_wayland_compositor ());
-
   framebuffer = clutter_paint_context_get_framebuffer (paint_context);
   cogl_framebuffer_draw_rectangle (framebuffer,
                                    overlay->pipeline,
@@ -385,8 +383,6 @@ meta_stage_update_cursor_overlay (MetaStage       *stage,
                                   CoglTexture     *texture,
                                   graphene_rect_t *rect)
 {
-  g_assert (meta_is_wayland_compositor () || texture == NULL);
-
   meta_overlay_set (overlay, texture, rect);
   queue_redraw_for_overlay (stage, overlay);
 }