Add workarounds for X servers doing out-of-bounds accesses.

Add workarounds for X servers doing out-of-bounds accesses.

Old X servers rely on out-of-bounds accesses when they are asked
to composite with a window as the source. They create a pixman image
pointing to some bogus position in memory, but then they set a clip
region to the position where the actual bits are.

Due to a bug in old versions of pixman, where it would not clip
against the image bounds when a clip region was set, this would
actually work.

The workaround added by this commit is to try and detect whether a
source drawable is actually a window without a client clip set. Such a
window will generally have a clip region that corresponds exactly to
the hierarchy clip in the server, whereas pixmaps will have a clip
region that is an exact match to the drawable.

When we detect such a window, we allow a fast path to run that would
normally be rejected due to the sources not completely subsuming the
composite region.

Fixed X servers should call the new function
pixman_disable_out_of_bounds_workaround() to disable the workaround.

This was reported in bug 22484.