metal: Added some support interfaces to Apple's Metal API (thanks, Caleb!).

Caleb Cornett's comments:

"A few weeks ago, Alex added a partial Metal API to SDL2:

I noticed it was missing a few features that would help Metal become a
first-class citizen in SDL, so I went ahead and wrote them! Here are the new

1. SDL_WINDOW_METAL flag for SDL_CreateWindow(). This allows the programmer
to specify that they intend to create a window for use with SDL_MetalView.
The flag is used to ensure correct usage of the API and to prevent
accidentally defaulting to OpenGL on iOS.

2. SDL_Metal_GetLayer(). This function takes a SDL_MetalView and returns a
pointer to the view's backing CAMetalLayer. This simplifies things
considerably, since in the current version of the SDL_Metal API the
programmer is required to bridge-cast a SDL_MetalView handle to an NSView or
UIView (depending on the platform) and then extract the layer from there.
SDL_Metal_GetLayer automatically handles all of that, making the operation
simple and cross-platform.

3. SDL_Metal_GetDrawableSize(). This function already exists in the current
SDL_Metal API (and is used behind-the-scenes for SDL_Vulkan_GetDrawableSize
on Apple platforms) but was not publicly exposed. My patch exposes this
function for public use. It works just like you'd expect.

Tested on macOS 10.14 and iOS 12.4."

Fixes Bugzilla #4796.
14 files changed