Fix bug 5297302 - Remember to release surface after starting drag

Forgetting to release it was "safe" in that GC would eventually
tidy it up anyway, but in the meantime it was possible for a user to
do lots of drag operations, racing ahead of the progress of the garbage
collector, and wind up with drags failing because we'd run out of
surface slots due to all the piled up stale drag shadows.

Change-Id: I7ac93b13cc9996dda04a404571fbc44cb4314694