Expose EAGLContext to MGLOpenGLStyleLayer subclasses

[1ec5]

MGLOpenGLStyleLayer should have a read-only EAGLContext property of type EAGLContext.

As seen in mapbox/ios-sdk-examples#111 (comment), it’s possible to integrate an SCNRenderer into an MGLOpenGLStyleLayer in order to render 3D SceneKit content into a style layer. However, it currently requires forward-declaring the MGLMapView.glView property, which is of type GLKView, so that an MGLOpenGLStyleLayer subclass can pass the GLKView.context property into +[SCNRenderer rendererWithContext:options:].

mapbox-gl-native/platform/ios/src/MGLMapView.mm

Line 202 in 6f0c992

@property (nonatomic) GLKView *glView;

MGLMapView.glView is an implementation detail that under ordinary circumstances shouldn’t be exposed to developers, especially not in light of a future Metal migration. However, MGLOpenGLStyleLayer is all about exposing OpenGL ES–specific implementation details, so it should have access to its EAGLContext.

The implementation of MGLOpenGLStyleLayer.EAGLContext would consist of returning self.style.mapView.glView.context. MGLMapView.glView’s declaration would need to be moved to MGLMapView_Private.h.

/cc @fabian-guerra @jmkiley