xwayland: Be smarter about picking seats for move/resize

xwayland: Be smarter about picking seats for move/resize

If we're going to move or resize an xwayland surface, we used to just
pick the first seat in the list for doing the move/resize.  Ideally we
can map from the XInput device doing the click to the corresponding
weston_seat, but that requires using xcb xinput, which isn't well supported.

Instead, lets use a simple heuristic that just picks the pointer that
most recently delivered a button event to the window in question.

Fixes: https://bugs.freedesktop.org/show_bug.cgi?id=73807