For Pixmap flips to have well defined outcomes the window must be contained by
the valid region if such region was specified.

The valid region is inserted as an argument to the check in window mode.
Setting this argument is missing in screen mode as well but we ignore it for now
and only add it to window mode.

It seems there are none or only very few clients actually making use of valid
regions at the moment. For simplicity we therefore just check if a valid region
was set by the client and in this case do never flip, independently of the
window being contained by the region or not.

Signed-off-by: Roman Gilg <subdiff@gmail.com>