Attach MetalLayer to Native View same as vulkan and metal backend.
This change assumes native window is already a CAMetalLayer. This is no different
for IOS and Mac.
Statically link SDL2 to make our samples (glTF Viewer, etc.) completely
standalone, and include missing resources (fonts and default environment
map for the IBL).
* Fix blanking windows not being drawn into on macOS
* Refactored to create swap chain info object; added fixes for resizing; added sample.
* CocoaGLSwapChain should inherit from Platform::SwapChain
* Link to SDL directly for multiple_windows example
* SDL apparently redefines main to SDL_main and requires C++ linkage
* Fix main to have SDL-approved signature
