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.