backend-vnc: use weston_region_global_to_output

Instead of directly converting damage from pixman_region32_t in
global coordinates to pixman_region16_t in local coordinates,
use weston_region_global_to_output() to convert to pixman_region32_t
in local coordinates and then convert again to pixman_region16_t
in the same coordinate system, using vnc_region32_to_region16().

Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>