This is done for consistency with the Android samples. Tested with
gltf_viewer against Vulkan, Metal, and OpenGL. Dragged window between
high DPI and low DPI displays.
Since resizing does not trigger the "NSView has changed" handler in
PlatformCocoaGL, it now must to detect a resize in order to make the
required call to NSOpenGLContext::update() as per the following
documentation.
https://developer.apple.com/documentation/appkit/nsopenglcontext/1436135-updateFixes#2528
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