Examples: ComputeSpriteBatch: add uniform buffer

[MirkoCovizzi]

Adds a uniform buffer to the compute SpriteBatch example to showcase how to use uniform buffers together with storage buffers.

[MirkoCovizzi]

This example, together with other ones, is a good test scenario for determining that SDL_Shadercross is working as expected. From HLSL to MSL there are no issues, but from GLSL to MSL the buffer indices are incorrect, which is addressed here https://github.com/libsdl-org/SDL_shadercross/pull/77