thin3d on Metal, plus a small render manager that owns frames, command
buffers, lazily started render passes and a per-frame push buffer.
Shaders are the same Vulkan GLSL the Vulkan backend gets, translated to
MSL through glslang and SPIRV-Cross, with the Vulkan binding numbers as
Metal buffer/texture indices. flip_vert_y makes Metal's clip space match
Vulkan's, so everything else follows the Vulkan conventions.
Adds GPUBackend::METAL (4) and builds the files on Apple platforms. Not
yet selectable: the SDL window setup, the UI choice and the GPU core
come next.
Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>
