Fix dragging on move/resize grabs

We need to use the grab_window for incoming events if we have an active grab.
cursor-tracker: Use a sync methodology for the cursor image

Rather than have MetaWaylandSeat do all the state management itself,
put the split between the root cursor and the window cursor in the
cursor tracker itself. We'll expand this to add a "grab" cursor in
the next commit, allowing us to force a cursor on grab operations.
cursor-tracker: Force the grab cursor on grab ops

This makes window moving have a 'window move' cursor, and similar.