x/y from XSizeHints are obsolete (and have been for a long time), so
instead use x/y from XWindowAttributes when USPosition|PPosition are
set.
The only reason we need to keep XSizeHints in our client_ctx is for
flags, so just add one to hints; consolidates sizehints and shrinks.
Additionally don't abuse PSize for XGetWMNormalHints() failures.