[cairo] text rendering fixes

[David Reveman]

I've fixed a few text rendering issues. This includes adding the new
font backend functions text_bbox and glyph_bbox, and I've made so that
show_text and show_glyphs should also be passed a source offset.

Using linear and radial patterns for text rendering should now be fully
working.

All these fixes are in my latest glyph cache patch:
http://www.cs.umu.se/~c99drn/cairo-glyph-cache-040520.patch

Do we want to make this glyph caching system part of cairo? My latest
text rendering fixes needs the glyph caching system. I could of course
move out the text fixes from the glyph cache patch but that would pretty
much mean that I would have to rewrite the fixes in a more inefficient
way.
 
-David