GPU: iOS requires stricter setup for depth stencils

[tadashibashi]

Hello, first off I want to say thanks for how awesome the gpu api is. Performance is amazing compared to OpenGL.

So I encountered a cross-platform quirk on iOS where I get a Metal assert about stencil info not being initialized when I'm only working with the depth buffer. On macOS Metal it works fine without setting the SDL_GPUDepthStencilState stencil parts. On iOS setting enable_stencil_test to true, compare_op to always true, and every stencil op in front_stencil_state and back_stencil_state to KEEP fixes it. Maybe some additional docs about this could help, or would it be a good idea to have reasonable defaults set on iOS?

[flibitijibibo]

Wonder if we just have to do the same thing for stencil that we did with depth write... here's where the stencil state is checked:

https://github.com/libsdl-org/SDL/blob/main/src/gpu/metal/SDL_gpu_metal.m#L1125-L1143

Though I also noticed that these values are NULL and not nil, which might make a difference? (I'm not an Objective-C expert, sorry...)

https://github.com/libsdl-org/SDL/blob/main/src/gpu/metal/SDL_gpu_metal.m#L1080-L1081

So either the initialized value is wrong or we just have to unconditionally initialize the front/back stencil descriptors. Spydog should be able to look at this soon, but if what I described fixes it I can merge a PR right away.