Request: GPU rendering backend

[ghost]

For text rendering using the GPU instead of the CPU.

https://code.videolan.org/videolan/libplacebo/-/issues/296

[TheOneric]

Personally I have no idea how to do this or how “generating a list of GPU commands“ is supposed to work and how it is different from libass rendering itself onto a given OpenGL/Vulkan surface. I have no idea how well versed others are with GPU-stuff, but to my knowledge no one is planning to do this in the foreseeable future.

Patches are welcome. Regardign where to plug a new rendering backend into, everything in the bitmap engine as well as this commit removing the old alternative font rasterizing backend might be helpful (though the GPU backend should probably be a runtime rather than build-time config): ef6cc02