r_viewfbo 0: skip the postprocessing/blending pass if there is nothing to blend.

r_viewfbo 0: skip the postprocessing/blending pass if there is nothing to blend.

May save some fps on low-end systems.

NOTE: Had to make sure that 2D rendering does not enable depth test,
as in r_skipblend mode, the main framebuffer Z values are written to as
part of regular 3D rendering. There appear to be no behavioral changes
from this, but we need to be careful here.

r_viewfbo 1 disables this logic (it differs in no other way from r_viewfbo 0)