CRT / post-process shader pipeline (libretro .glsl + .slang) with shader downloader

[JRickey]

Summary

Adds a post-process shader pipeline to the SSB64 PC port: libretro
single-file .glsl, .glslp presets, and .slang/.slangp shaders
are normalized, transpiled (GLSL → SPIR-V → GLSL/HLSL/MSL via glslang +
SPIRV-Cross), and run as a multi-pass chain across all three Fast3D
backends (OpenGL, Metal, D3D11). Includes the in-game shader picker,
the two-phase fetch+install shader downloader, runtime diagnostics, and
the bundled scanlines / crt-lottes shaders.

D3D11 bring-up fixes (this round)

Every shader worked on OpenGL and Metal but failed on Windows/D3D11.
Root-caused via staged runtime bisection and fixed:

1. VS↔PS signature register-order mismatch — the root cause of the
   black screen for every shader that compiled. kStockHlslVertex
   declared SV_Position before TEXCOORD0; SPIRV-Cross emits the
   fragment input struct with the user varying first and SV_Position
   last. D3D11 links VS-output→PS-input by hardware register index
   (declaration order; SV_Position is handled separately by the
   rasterizer), so TEXCOORD0 mismatched and the PS sampled garbage
   UVs → black. Reordered VOut to mirror the SPIRV-Cross signature.
   GL/Metal were immune (SPIRV-Cross generates both stages there).
2. Overlapping HLSL sampler registers — non-schema samplers (e.g.
   a libretro shader's bezel/LUT) all defaulted to t0/s0,
   colliding with Source; FXC hard-errored (X4500) and rejected the
   shader. They now get unique registers above the schema range.
3. Stale scissor — the D3D11 post-process draw inherited the game
   path's ScissorEnable=TRUE + game-FB scissor rect, clipping the
   pass on the window-sized mDstFb. Now sets a full-destination
   scissor.
4. MSAA resolve target — mGameFbMsaaResolved was allocated under
   a narrower condition than ComposeFinalFrame consumes it; with
   MSAA on + a shader active at matched resolution it was never
   created. Allocation now mirrors the consume condition.

Verified on Windows/D3D11 (NVIDIA, MSAA 4x): bundled scanlines /
crt-lottes and libretro shaders (crt-Cyclon, etc.) render
correctly; OpenGL and Metal paths unchanged.

Clean-room

Authored against public libretro shader-spec documentation, the
glslang/SPIRV-Cross APIs, and public D3D11/HLSL signature-linkage
docs. No RetroArch or GPL-licensed shader-runtime code was referenced
or copied.

Submodule

Rides a libultraship pointer bump (claude/postprocess-shaders on
the fork) carrying the four D3D11 fixes above plus the transpiler /
chain implementation.

🤖 Generated with Claude Code