This PR introduces support for ARKit to the iOS version of Godot.
ARKit is Apples Augmented Reality platform.
This PR brings in support for ARKit 1.0 and implements a few ARKit 2.0 features.
It requires iOS 11 to run but should not prevent Godot from running on older versions as long as ARKit remains unused.
This is a new singleton where camera sources such as webcams or cameras on a mobile phone can register themselves with the Server.
Other parts of Godot can interact with this to obtain images from the camera as textures.
This work includes additions to the Visual Server to use this functionality to present the camera image in the background. This is specifically targetted at AR applications.
