[cairo] Use 8-bit colour values internally

[Billy Biggs]

Jason Dorje Short (jdorje at users.sourceforge.net):

> Billy Biggs wrote:
> > The 16-bit representation of colours (cairo_color_t's shorts) is
> > only used by the fill_rectangles implementation for the image and
> > Xlib backends.  However, neither really needs this much precision:
> > pixman and X both convert back to 8 bit internally by shifting.
> 
> In the pixman code in the patch it looks like pixman is using 16-bit
> values...

  iccolor.c just shifts them back to 8-bit and then does the fill.

  -Billy