D3D12: Fix HDR rendering pipeline, resource states, and overlay support

Add separate HDR pipeline states (pipe_blend_hdr, pipe_noblend_hdr,
pipe_font_hdr) for R10G10B10A2 render targets and select the correct
pipeline based on the current render target format, tracked via a new
current_rt_format field.

Fix resource state management: create render target textures in
RENDER_TARGET state instead of PIXEL_SHADER_RESOURCE, add mipmap
generation with zero-size level guards, move resource transitions
to correct positions in the frame loop, and add missing back buffer
transitions after HDR compositing.

Fix HDR compositing: always use R8G8B8A8_UNORM for the intermediate
back buffer regardless of shader preset format, set root signature
before descriptor table binds, populate source/output size in the
HDR UBO, and set the blend pipeline after menu texture upload.

Extend HDR back buffer compositing to handle overlays in addition
to the menu (D3D12_ST_FLAG_OVERLAYS_ENABLE).

Only request GPU descriptor handle for shader-visible descriptor
heaps to avoid D3D12 validation errors.

Author: MajorPainTheCactus