Update FBO binding semantics

-Now track entire fbo_t struct instead of just the FBO handle
-Add explicit fbo_t struct for the screen
-Deprecate most uses of GL_BindFBO in favor of R_BindFBO
Significant cleanup

-Fixed bugs in SteamVR code path
-Moved most texture blits to VBO based path
-Removed dead cruft from VR code paths
-Add r_hud_separate_fbo to control if hud and view are drawn in two
different buffers while outside of VR mode
-Move VR compositing into R_VR_EndFrame()
-R_VR_Present now only copies final composited image into screen
-Significantly improve texture blit / quad draw interface in
r_postprocess
-Eliminate dead code in r_antialias
Shader update