xfixes: Restore monitoring of animated cursors

This was broken by:

    commit aa6651f8
    Author: Adam Jackson <ajax@redhat.com>
    Date:   Thu Jun 8 16:23:12 2017 -0400

        xfixes: Remove the CursorCurrent array

As of that change we look up the current cursor dynamically instead of
trying to track every time it's set through ->DisplayCursor. That would
work, except the 'bits' of an animated cursor is a transparent 1x1
pixel. So now, look up whether there's an animated cursor, and use its
current frame if so.

Signed-off-by: Adam Jackson <ajax@redhat.com>