render: Add SDL_RenderGeometryRawColor8Bits for 8-bit color vertex input

[alattanzio]

Adds SDL_RenderGeometryRawColor8Bits to allow passing vertex color data as 8-bit components (SDL_Color) directly.

Key Changes:

• New function SDL_RenderGeometryRawColor8Bits for 8-bit vertex input.
• Removes the need for manual float/packed-int conversion by the user.
• Consistent with existing SDL_RenderGeometryRaw logic.

[slouken]

This is an easy wrapper you can have in your own code. I don't think we'll add this to the SDL API surface.

[alattanzio]

This is an easy wrapper you can have in your own code. I don't think we'll add this to the SDL API surface.

I agree. I created this PR because I thought this wrapper might help other developers who are migrating from SDL2 to SDL3, like I’ve been doing. :)