The old condition happened to work, because SFLAG_INTEXTURE was not
set(we're loading it), and neither was SFLAG_INDRAWABLE(otherwise
INTEXTURE would be set too). If the fbo INDRAWABLE == INTEXTURE is
replaced by INDRAWABLE == INSRGBTEX this doesn't hold true any longer
because SFLAG_INDRAWABLE is set, but not used because the drawable
readback is never used for fbos.