window: Fix crash in input_set_pointer_image when cursor is special

Certain circumstances may lead to the "force" clause in
input_set_pointer_image() being reached when the current cursor
is blank or unset.  These are special cursors that don't have
images, and they need to be handled differently than image cursors.

This patch puts the special cursor handling in its own function and calls
it from both places that need it.  Previously only the frame callback
handler did this correctly.

Signed-off-by: Derek Foreman <derekf@osg.samsung.com>
Reviewed-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>