Fix OpenGL stage-clear wallpaper rendering upside-down

[JRickey]

Summary

• 1P stage-clear frozen-frame wallpaper rendered upside-down on OpenGL only — D3D11 and Metal already worked.
• port_capture_game_framebuffer was force-flipping rows on OpenGL on the assumption that glReadPixels uses bottom-left origin. That's true of raw GL, but mGameFb is created with opengl_invertY=true (libultraship/.../interpreter.cpp:5893), so the engine pre-negates vertex Y at draw time (interpreter.cpp:2761). The OGL framebuffer's bottom row is therefore already the N64 top row, and the wrapper's "compensating" flip was inverting an already-correct buffer.
• Confirmed via a short-lived SSB64_FB_FLIP=always\|never\|auto env-var harness from a single Debug build — only never produced an upright wallpaper. Locked in as the unconditional default.
• D3D11 / Metal ReadFramebufferToCPU walks textures in native top-left order with no analogous flag, so they were correct without any flip and remain unchanged.

Test plan

• OpenGL backend: 1P → any stage → fight to stage clear → wallpaper renders upright (was upside-down on main).
• D3D11 backend: regression check — stage-clear wallpaper still upright (no behaviour change in code path).
• Metal backend: regression check — stage-clear wallpaper still upright.

Notes

• See docs/bugs/opengl_fb_capture_y_flip_double_invert_2026-05-03.md for full root cause + audit hook.
• mGameFbMsaaResolved is the only LUS-managed FB built with invertY=false. Capture only reaches it on the OGL+MSAA+non-matching-viewport sub-config, which we couldn't trigger in the A/B session — if a future report describes upside-down only with MSAA on, this is the next likely place to look.

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