Bug 845874. Switch to Y-X banded regions. r=roc

Previously our region code was just a simple y,x sorted list of
non-intersecting rectangles. This can cause us to have simple regions
represented in a complex unoptimizable way.

Switching to pixman regions gives us a canonical region implementation.

There are some cases when this can cause performance regressions.

For example, with the old region code we end up with this region:
http://people.mozilla.org/~jmuizelaar/region-pre.html
which is represented like this:
http://people.mozilla.org/~jmuizelaar/region-post.html
with the new code.

We call SimplifyOutward(4) on this. With old regions we can't simplify it so we
end up taking the bounds and get 1 rect. With the new regions we have only 3
rects to start and so we do nothing. The difference between 3 rects and 1 rect
cause D2D to do a PushLayer() instead of a ClipRect() and that seems to be the
causes for the regression.

--HG--
extra : rebase_source : 65e0d29d67b51a3780448eaecfde33dbcb6b99b1