Merge: gamnit VR: steroscopic view and head tracking on Android

This PR adds support for VR to the _gamnit depth_ framework. By importing `gamnit::vr`, all drawings are duplicated for a stereoscopic view and the `world_camera` orientation is updated from head tracking data.

VR is applied to the `model_viewer` app for testing only. The UI should still be updated for a nicer VR experience.

This is done by simple refinement, which allows for easy (but dirty) conversion to VR using `nitc ... -m lib/gamnit/depth/vr.nit`. However this does not allow to easily switch between VR and a classic view. This could be improved as needed in the future by using a subclass of `EulerCamera` and a few conditions in `frame_core_draw`.

Pull-Request: #1923
Reviewed-by: Jean Privat <jean@pryen.org>