Use a pair of NSSets to manage the retain/release lifecycle of layers.

[julianrex]

(Obj-C) Layers are not retained at all by the SDK, but the C++ Layers can reach back to them (especially as custom layer contexts). I think there needs to be some mechanism that holds on to the Obj-C layers, regardless of what the client app does.

This PR adds a set (to MGLStyle) to which these layers are added, purely to manage the retain counts. When rendering starts, this set is copied, so that their lifetime is guaranteed (and so those contexts was become garbage).

(It's possible that the initial copy might need to move from rendering start to being triggered Map::Impl::onUpdate())

(This addresses #11143)

[asheemmamoowala]

These changes do not address the scenario in testOpenGLLayerDoesNotLeakWhenStyleChanged. (It's possible the test is passing, but I believe the underlying issue is not addressed here).

When the style is changed with a call like [mapView setStyleURL:styleURL], the core layer is eventually cleaned up, but there will not be a corresponding call to [MGLStyle removeFromManagedLayers].

One possible way to capture this case is to handle the StyleObserver::OnStyleLoaded callback and check if any layer ids have been removed. To implement this, the NSMutableSet will need to be changed to an NSMutableDictionary so that the layer identifier does not have to be compared against the id in the core layer, which may already have been deleted.

[asheemmamoowala]

This needs to be invoked from [MGLStyle layerFromMBGLLayer] as well, to capture internally created style layers.

[asheemmamoowala]

Does the use of these properties remove the need for tracking the peer object in core for style layers? For layers created in the SDK, [MGLStlye layerFromMBGLLayer] could be used to lookup this set instead of querying core

[julianrex]

The latest changes have evolved somewhat from the initial PR:

• An MGLStyle still holds on to MGLOpenGLStyleLayers (but now restricted to just that type).
• The MGLMapView has an object that manages the retain/release for the layers that a referenced from RenderLayer (in fact their Impls). This object uses a lifetime mechanism to know when to remove the object from its registry. The lifetimes are reset (using the MGLStyle's set) at the beginning of the render process, and decremented at the end. This handles the case where the style can be changed mid-flight.
• I've had to add a deallocation callback to mbgl::style::CustomLayer which isn't ideal - so that the MGLOpenGLStyleLayer can clear its unmanaged rawLayer pointer (via the peer object). As I understand it this affects all platforms, e.g. QT, Android - I have not yet made the corresponding change there.
• Additional tests have been created and existing ones beefed up to hopefully catch all the possible scenarios.

[julianrex]

Closing in preference of #11553