Generating cubemaps set backEnd.viewParms.isMirror = qtrue while the
loading screen said 'loading... maps/q3dm1.bsp' and it just stayed like
that until done loading (no additional messages or item icons) because
all 2D drawing was culled due to flipped culling for isMirror.
This was noticed because a recent commit fixed RB_ShowImages() to be
drawn to the screen instead of draw into renderFbo and never blit to
the screen. Now the loading screen draws over it as expected.
Mentioned commit: a81df34905
"OpenGL2: Fix border for cg_viewsize using HDR/FB-MSAA w/postProcess"
972635ea5a