Expose API so we can draw to Compositor layer

[shafy]

Drawing to the compositor layer instead of the eye buffer is useful for a couple of things. An important use case is to render text more clearly. They already provide scripts for this in their Unity SDK called OVROverlay.

[m4gr3d]

In order to expose this feature in Godot, we could extend Godot's Viewport (e.g: OvrViewport) and have it backed by an ovr compositor layer.