NEW: HMD tile config support
FIX: const change: DrawContext is pased as non-const to Renderer and Camera to allow for changes (for instance to viewport, needed for rift up-scaling)
FIX: Viewport size in side-by-side stereo during overlay cyclops step
NEW: Alt+mouse move now controls head rotation in KeyboardMouseCameraController
NEW: (experimental) 2x upscaling in rift renderer
UPDATE: Rift related camera changes