bug#35468: [PATCH] Refactor draw_glyph_string on X and w32

[mituharu@math.s.chiba-u.ac.jp]

>> And what does glyph_image_uses_mask hide?  AFAIU, the current code
>> simply looks at s->img->mask, and if so, why do we need an interface
>> for that?
>
> I was thinking that since AFAIU the Cairo drawing doesn't set
> s->img->mask it wouldn't make sense, from an interface POV, to check it
> directly. I suppose it doesn't really matter in that case, and it would
> be faster to just check s->img->mask even if the backend doesn't use it.

IMO, image support for cairo is still premature and needs some
restructuring.  It does not support postprocessing (:conversion
ALGORITHM), mask removal (:mask nil), or image-mask-p.  Bitmaps
for some image format are not stored in the premultiplied alpha
format that cairo expects.

All of them are supported in the Mac port and I set dummy
s->img->mask there (not containing the actual mask bitmap data)
if the image has an alpha channel.  Probably setting a bitfield
for the existence of mask (alpha channel) when creating image
data would work as an alternative way.

				     YAMAMOTO Mitsuharu
				mituharu@math.s.chiba-u.ac.jp