tests/kms_cursor_crc: reduce cpu work on restoring image

on creating reference images for crcs re-render only parts
which have changed.

Signed-off-by: Juha-Pekka Heikkila <juhapekka.heikkila@gmail.com>
Reviewed-by: Vinod Govindapillai <vinod.govindapillai@intel.com>