Descriptor sets

hardcoded in SDL_gpu
for graphics pipelines there are always 4 of them:
- 0 are vertex resources (first samplers, then storage images, then storage buffers)
- 1 are vertex uniform buffers
- 2 are fragment resources (again samplers, storage images and storage buffers)
- 3 are fragment uniform buffers
compute has 3 sets:
- 0 are read-only resources (samplers, storage images, storage buffers)
- 1 are write-only resources (storage images and storage buffers)
- 2 are uniform buffers

Cycling

due to the nature of in-flight frames, resources might still be in use in a previous frames
to make up for this, many SDL_gpu APIs have a cycle parameter
if set to true, SDL will provide an unused resource if the previous one is still in use