If two cursor buttons are pressed at the same time, the client will now
be notified of the second button press.
The main reason for not sending the concurrent presses was due to an
early return in dispatch_cursor_button if a seatop is in progress. This
patch makes it call seat_pointer_notify_button prior to returning. But
it also has to make sure there's not a mismatch in events such as a
release without a press.
Prior to this patch, the down seatop would send press and release events
in its begin and finish functions. No other seatops did this. A press
event would be sent prior to starting tiling drag, but never an
associated release.
After this patch, no seatops send their own press or release events. We
send them prior to calling the seatop begin functions, then the first
part of dispatch_cursor_button handles all presses during seatops and
when releasing the seatop.
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