gpu: Add instanceCount parameter to SDL_GpuDrawPrimitives()

[Akaricchi]

Partially fixes #231

[flibitijibibo]

Should be okay if CI is happy

[thatcosmonaut]

I'd really like to see an indirect variant of this and a functioning test before merging.

[Akaricchi]

The plain IndirectDrawCommand already has instancing parameters. If anything, this PR makes the non-indirect one more consistent with it.

SDL/include/SDL3/SDL_gpu.h

Line 498 in e099043

typedef struct SDL_GpuIndirectDrawCommand

typedef struct SDL_GpuIndirectDrawCommand
{
    Uint32 vertexCount;   /* number of vertices to draw */
    Uint32 instanceCount; /* number of instances to draw */
    Uint32 firstVertex;   /* index of the first vertex to draw */
    Uint32 firstInstance; /* ID of the first instance to draw */
} SDL_GpuIndirectDrawCommand;

I'm not sure why or where do you want a test for this. The only test we have in the repo is the spinning cube, which certainly doesn't cover the entire API surface. Do you want me to add an example to the SDL_gpu_examples repo?

[flibitijibibo]

Do you want me to add an example to the SDL_gpu_examples repo?

That's probably the way to go. We already have an instancing sample so we can just rip that off.

[thatcosmonaut]

I reviewed and all the backends do in fact take an instance count for the indirect variant so this does make it more consistent. Just need to use it in SDL_gpu_examples and we're good to go.

[flibitijibibo]

Getting this in now because the weekly rebase is going to take all day and this is mostly an obvious change - we still expect an example though, so I've filed that at TheSpydog/SDL_gpu_examples#31