Don't use zero-sized viewport as disengaged state.
Don't use zero-sized viewport as disengaged state.

Zero-sized viewport is perfectly valid state and it causes assertion
failures on contexts without default framebuffer (e.g.
WindowlessCglApplication).

ashimidashajia/magnum,ashimidashajia/magnum,MiUishadow/magnum,MiUishadow/magnum,DerThorsten/magnum,DerThorsten/magnum,ashimidashajia/magnum,MiUishadow/magnum,ashimidashajia/magnum,DerThorsten/magnum,MiUishadow/magnum,DerThorsten/magnum,MiUishadow/magnum,MiUishadow/magnum,ashimidashajia/magnum,DerThorsten/magnum