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
