SDL_BeginGPURenderPass2

[thatcosmonaut]

Occlusion queries have exposed that Metal wants those to be part of the render pass descriptor, so it's probably time to think about the fabled coming of BeginRenderPass2.

Things that should be part of the SDL_BeginGPURenderPass2 API:

• Support storage buffer/texture writes from render passes
• Allow depth layer/level higher than 255
• Occlusion queries

If anyone else has something they want render passes to do, speak now or forever hold your peace.