Add Depth32F, Depth32F_Stencil8 pixel formats, no Depth24_Stencil8 on iOS

On iOS, Metal does not support Depth24_Stencil8 pixel format (it's marked UNAVAILABLE), and trying to use it results in a compiler error. ## Description This pull request adds the pixel formats `SDL_GPUPIXELFMT_Depth32F` and `SDL_GPUPIXELFMT_Depth32F_Stencil8`. These formats are supported on Metal, Vulkan, and DX12, and have been supported by both desktop and mobile GPUs for years. This pull request also makes trying to use `SDL_GPUPIXELFMT_Depth24_Stencil8` return an error on iOS. ## Existing Issue(s) [Issue #5](https://github.com/icculus/SDL/issues/5)