Remove workaround for a bug in the 1.6 X server.

Remove workaround for a bug in the 1.6 X server.

There used to be a bug in the X server where it would rely on
out-of-bounds accesses when it was asked to composite with a
window as the source. It would create a pixman image pointing
to some bogus position in memory, but then set a clip region
to the position where the actual bits were.

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. So when the pixman bug was fixed, a workaround was
added to allow certain out-of-bound accesses.

However, the 1.6 X server is so old now that we can remove this
workaround. This does mean that if you update pixman to 0.22 or later,
you will need to use a 1.7 X server or later.