Shader fixes & improvements

-Spun off GL_ARB_shaderobjects interactions to r_shaderobjects.c
-Moved OVR specific shader functions into vr_ovr.c
-Added bicubic filtering distortion shaders for the Rift -
vr_ovr_bicubic
-Added 2x supersampling support for VR render path - vr_supersampling
-Fixed a bug in libovr.cpp
Add menus for supersampling and bicubic filtering
Move towards more generic antialiasing support

-Committed now for forward compatibility with future releases