Threaded OpenGL rendering

[rcari]

Rendering should occur from another thread than the GUI thread. Currently, the GUI thread places all the draw calls.
On heavy scenery, this may cause loss of reactivity.
Another issue is for stereo rendering which requires continuous updates, this can only be reasonably achieved using a dedicated rendering thread alterning left/right eye renders.
For standard rendering, the views should be updated only on receiving an event requiring it.

[rcari]

All OpenGL buffer objects should be implemented using double buffering. All buffer uploads should be implemented as Tasklets.