Mac Glass: use Prism for managing FBO

Currently Mac Glass uses a native implementation of its own FBO, which it uses to provide content when the View layer wants to paint. Strictly speaking, we could use the FBO architecture used by Prism itself. I imagine Prism would need to change, so that it draws into 2 FBOs (double buffer) and provides the already drawn FBO to Glass. The switch between the FBOs would need to happen inside View.lock/unlock

This would allow us to simplify Mac native code greatly.