shell: Update focused surface pointer coordinates in popup grab

shell: Update focused surface pointer coordinates in popup grab

The commit 'input: Send leave and enter pair when the surface moves
under the cursor' introduced focused surface local pointer coordinates
to keep track of if a surface had been moved or transformed in a way
that the pointer posititon relative to that surface would change.
Update these coordinates also for the popup grab as otherwise every
pointer motion during a popup grab results in leave and then enter
events.

Signed-off-by: Jonas Ådahl <jadahl@gmail.com>
Tested-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>