[Renderers/SDL3] Remove unused includes

[kraxarn]

The "SDL3/SDL_main.h" header should only be included with the application entry point, which a renderer probably shouldn't include. In my setup, when included with the renderer, I get an error similar to:

Undefined symbols for architecture arm64:
  "_main", referenced from:
      <initial-undefines>
ld: symbol(s) not found for architecture arm64

Simply removing it makes the error go away, and I can't find it being used anywhere either.

The "SDL3_image/SDL_image.h" header also forces any application that uses the renderer to include SDL_image, which isn't even being used from the renderer. However, it is used in the example, so I feel like it makes more sense to have it there instead.

Thank you! ☺️