[Cairo] libXc is dead

[Carl Worth]

I've just committed changes to incorporate all of the functionality
from libXc into libcairo. So, Cairo will no longer link against libXc,
(and we can all ignore the Xc module from now on).

I've also moved the Cairo internals away from most of the X datatypes,
(XFixed, XPointFixed, etc.), so it's getting closer to having honest
support for multiple backends now.

The next task in the backend-rework is to change cairo_surface to call
backend-specific functions through function pointers rather than just
testing surface->type and calling directly.

Then, the X-specific portions of the implementation and header file
can be moved into their own files. And that will provide a nice model
for dropping in other backend implementations as well.

-Carl