[cairo] Use _cairo_rectangle_intersect in more places for intersection checking

[BJörn Lindqvist]

Hello,

Here is two patches, the first modifies _cairo_rectangle_intersect()
so that TRUE or FALSE is returned whether the rectangles intersect or
not. Just like about five million other rectangle intersection
functions work.

Which is important for the second patch that modifies all
_cairo_*_get_image() functions so that they now use the
_cairo_rectangle_intersect() function for doing intersection checking
instead of spelling it out themselves.

The benefit is that quite a few lines of code are removed. I also
added some docstrings to some touched functions because I found it
hard to tell what those functions were doing.


-- 
mvh Björn
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