GPU: `VERTEX | INDEX` buffer usage is forbidden by docs but works

The docs for SDL_CreateGPUBuffer say: > Note that certain combinations of usage flags are invalid. For example, a buffer cannot have both the VERTEX and INDEX flags. This is inconvenient for certain use cases, e.g. it prevents a 1:1 correspondence of gLTF buffers with SDL buffers. It isn't enforced by any debug checks, and `SDL_GPU_BUFFERUSAGE_VERTEX | SDL_GPU_BUFFERUSAGE_INDEX` does work on the devices I've tried it on. I'm hoping it's possible to either confirm that this usage is OK and remove that line from the docs, or else enforce it as a debug check.