We shouldn't just discard the mask if the user requests that we render
the glyphs through a low bitdepth mask - and in doing so we should also
be careful not to improve the bitdepth of that mask (since we don't take
into account the extra quantisation desired).
Testcase: render-glyphs
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Chris Wilson