gpu: add dummy pl_gpu backend
This is basically a software emulated `pl_gpu`, which does all texture/buffer operations in host memory using CPU instructions. Attempting to somehow soft-emulate shaders or rasterize is _way_ out of scope for this dumb thing, so we don't support `pl_tex_blit` or `pl_pass` at all. Literally the only point is to let users generate "faux" shaders, while keeping track of the generated LUTs etc.
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