[Davey-Hughes]

Guidelines

1. Rebase before opening a pull request
2. If you are sending several unrelated fixes or features, use a branch and a separate pull request for each
3. If possible try squashing everything in a single commit. This is particularly beneficial in the case of feature merges since it allows easy bisecting when a problem arises
4. RetroArch codebase follows C89 coding rules for portability across many old platforms check using C89_BUILD=1

Description

Since 0e7a56f each GPU index (Vulkan, OpenGL/EGL, Direct3D 10/11/12, Metal) remembers the device it was chosen as, in video_gpu_name_*. Changing the index in the menu left the old name saved, so on the next driver init video_driver_gpu_index_resolve() found that name at its old position and went back to it: the log says the old GPU is now #1, not #2; using it and the new choice is ignored. On the EGL contexts (wayland, x/egl, drm) the menu value reverts as well. The only way out was to set the index to 0 first. This is on master only; 1.22.2 predates it.

Left, Right, the dropdown and reset now save the name of the device at the chosen index, through a new video_driver_gpu_index_select(). Index 0 clears the name, and an index past the end of the current list leaves it alone, so a stale out-of-range index still follows its device. Left no longer sets the index to -1 on an empty device list.

Testing

samples/gfx/gpu_index builds the production resolver, the Left/Right callbacks and the write-handler case from source and checks, for every API, that a menu change survives a driver reinit and that an out-of-range index keeps its device. It runs in the samples-gfx job, on Linux with ASan/UBSan and -std=c89, and under mingw + Wine. Run against master's callbacks, the same change-then-reinit sequence keeps the old GPU on Vulkan, GL, D3D11, D3D12 and Metal.

• Linux (gcc, ASan/UBSan, C89 flags) and mingw-w64 under Wine: pass
• Native Windows 11, MSYS2 UCRT64 gcc 16.2: pass
• The four touched objects build clean with C89_BUILD=1
• tools/ci_local.py selftest and tools/ci_apt_check.py pass

On a desktop with an RX 7900 XT, a Radeon 610M iGPU and llvmpipe: start on index 2, press Left once on Settings > Video > Output > GPU Index, quit, relaunch.

	master	this PR
Vulkan, llvmpipe → 610M	llvmpipe again: GPU "llvmpipe (LLVM 23.1.3, 256 bits)" is now #2, not #1; using it.	610M
Vulkan with the iGPU listed first (MESA_VK_DEVICE_SELECT=1002:13c0), llvmpipe → 7900 XT	llvmpipe again, same warning	7900 XT
GL, egl_x context, 7900 XT → 610M	7900 XT again, and gl_gpu_index back to 2	610M, index stays 1

Relaunching the 610M config with llvmpipe listed first (MESA_VK_DEVICE_SELECT=10005:0000) still picks the 610M, now at index 2, with the is now #2, not #1 warning. These ran under Xvfb, which can present only from llvmpipe, so the table shows the device each driver selected, from the log.

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