[cairo] null backend

[David Reveman]

I wanted to compare the OpenGL output performance to the "ultimate"
output performance, so I created a simple null output backend for cairo:
http://www.cs.umu.se/~c99drn/cairo-null-surface-040518.patch

I know that no optimizations have been done to cairo yet, but I think
the results are still quite interesting, especially when discussing the
performance of the OpenGL backend.

Here are some results from running the cairogears program on a P3 1Ghz
with a geforce fx.

(glitz 0.1.3, anti-aliased GL output and cairo CVS with my glyph cache
patch and the null backend patch)

Numbers are FPS.

        IMAGE      GL      NULL
TRAP        4      55        79
GRAD        3      45        72
COMP        2     490      7010
TEXT        6      21        27
TEXT2      34     410       640
SHADOW      1     170      2530

 
-David