ported from -js: eb6c6596c6a7a61363d30356674e0002153b1d19
`altitude` was a terribly-named variable that was used to indirectly
control the fov. This should eliminate some confusion.
`altitude` was equivalent to `cameraToCenterDistance / height`
* [ios, macos] Replaced custom style layer API with MGLOpenGLStyleLayer
Replaced the custom style layer API on MGLMapView with an equally unsupported MGLOpenGLStyleLayer API that nonetheless is consistent with the broader runtime styling API and is compatible with macOS.
Fixed an unrecognized selector crash when wrapping a layer of unrecognized type coming from mbgl.
* [macos] Added lime green layer demo to macosapp
Reprised the demo removed from iosapp in #5091.
* [ios, macos] Rationalized MGLOpenGLStyleLayer API
MGLStyle now strongly references any MGLOpenGLStyleLayer object that’s added to it, in order to prevent pointers from going stale and make it easy for layer drawing code to get more information about the map view. Replaced the MGLOpenGLStyleLayer callback blocks with overridable instance methods. Added internal documentation for each method. Subclassed MGLOpenGLStyleLayer as LimeGreenStyleLayer inside macosapp.
Consolidated -addToMapView: into -addToMapView:belowLayer: to ensure that MGLRedundantLayerException gets raised even if the layer is being inserted rather than added to the bottom of the stack.
* [core] Clarified that rendering happens on the main thread
* [ios, macos] Fixed removing and re-adding MGLOpenGLStyleLayer
Don’t allow index-based layer removal to circumvent -removeFromMapView:, which MGLOpenGLStyleLayer relies on to synchronize the style’s array of MGLOpenGLStyleLayers. When obtaining an MGLOpenGLStyleLayer, get the instance already added to the style instead of creating a new one to wrap the underlying CustomLayer.
