Rendering improvements

-Moved to rendering into a single backbuffer.
-Moved to applying bloom and color blending in a single pass across the
entire backbuffer.
-Allow disabling the barrel distortion shader using vr_ovr_distortion
-Changed shaders so they are more inline with Oculus provided shaders
Fixes for rendering
-Fixed issue with crosshair size being scaled incorrectly
Fixes
-vr_ovr_driftcorrection stays set even if there is an error enabling it
-Better detection of if a device is available or not
-Improved responsiveness of HUD bouncing