clients/window: Fix animated cursors

Since commit 992ee045 we create a new surface when we update the cursor
image. This broke animated cursors by discarding any existing frame
callback used for timing, and moving the setup for frame callbacks to
after the commit on the pointer surface.

To fix this we need another surface commit for the frame callbacks, but
this alone is not enough to fix the regression, as a lingering kludge
intended to fix problems when reusing the pointer surface is no longer
working as intended.

Since we no longer re-use the same surface, we can delete the old surface
on pointer exit, along with any callbacks set on it. Then a frame callback
will be recreated naturally. This lets us remove the now broken kludge
from the past and restore animated cursor functionality.

Signed-off-by: Derek Foreman <derek.foreman@collabora.com>