[cairo] Does cairo use backend-specific support for groups?

[Nikita Zlobin]

I tried to implement text shadow by prerendering text and overlaying it
multiple times (hoping to get better performance, than just drawing
pango layout each time).

Main surface is X window.

Tried 3 methods:
- image surface as intermediate, manually painting it
with transparency and offset,
- cairo groups,
- xpixmap surface for intermediate, overlaying with xrender composite
  call.

Can't remember performance with groups - only remembering, they are
faster than image surf, however same is for manual xrender call.

Unfortunally, using xpixmap-based surface as source did not work - I
tried to work, but I got emptyness (black fill). Never tried Cogl
though, since I use cairo from distro, which is at 1.16.0.

I already was told there, that cairo uses intermediate image for groups.

Q: Do cairo groups use backend-specific type of surface, such as
xpixmap with X11 as main backend?
For now I'm not finally sure if manual xrender'ing two pixmaps is better
than using cairo groups, I only know - it's certainly better than using
image surfaces as source.

For opengl-based backends this could be opengl texture, which also
could be overlaid directly if used as source to draw to same type of
surface.