improvement on stereo rendering (but not Occulus....)

Follow up to Brad's recent optimization to bucket all the entities of the scene and finally create a single gpu::Batch for all the meshes of all the enitites. 
I introduced a Side enum in the call to Application::displaySide, it can be MONO, STEREO_LEFT or STEREO_RIGHT. THis is used in the Model::endScene() call to actually skip completely the Batch creation for the right side. 
This saves us  a significant amount of time on the overall entitiy pipeline (~20%)

Next, need to do the same for occulus...