[cairo] 256 color quantization and paletting

[Andrew McRae]

Comrades,
I have some patches for the following changes:

 - Octree based quantization and paletting
 - PNG patches to allow the above to produce
   256 (or 16 or whatever) color paletted PNG files.
 - A JPEG backend

Not sure of the best way to send these - just
a patch will do?

The PNG changes introduce an optional set of
parameters to tell the PNG code various things,
such as whether to palette or not, and
whether the default compress level and
filter options are overridden. I did this
by adding a cairo_surface_write_to_png_param()
function that takes a parameter structure. Thus
it is backwards compatible.
The jpeg backend uses the commonly available jpeg
library.

Cheers,
AMc