Make some enums ints

[flibitijibibo]

I'm not super sure about IndexElementSize...

SDL_GpuIndexElementSize: Should we just make this an int?

Definitely want to do this for sample count:

SDL_GpuSampleCount: same (but I don't feel strongly about either of these, I know that the lower level tends to use enums for these anyhow)

[flibitijibibo]

Wanted to do this one today but it's a bit too invasive... I'm going to make the call now that IndexElementSize is staying since raw ints are an invitation to do the math wrong (i.e. "did they mean bits or bytes?"), sample count I could go either way - @thatcosmonaut, @TheSpydog, will leave this up to you depending on what each backend prefers.

[flibitijibibo]

Looking through the API support coverage, the enum values for sample count are probably the way to go - we could maybe add 16X but it's not a dealbreaker.