present: Check valid region in window mode flips

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 for equality of
regions.

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