[cairo] Create a new backend

[german rodriguez]

Hello,

I want to know how could I create a new backend for cairo. I was
looking at the code and apparently I should provide the functions for
_cairo_surface_backend structure. I looked into xlib backend
implementation but I couldn't understand how cairo access to the
backend for drawing things. I didn't find any documentation which
allow me to understand it, could you instruct me in how to do it??

For example, if had a set of interfaces to implement and what that
interface had to do?

I wasn't able to find in cairo_xlib_* anything like a call to a
function XDrawLine(...) which would mean that the function was used to
draw into a X11 Drawable.
-- 
German