backend-vnc: rename vnc_convert_damage to vnc_region_global_to_output

Align the function name and arguments of vnc_convert_damage() with
weston_region_global_to_output(). It does not support rotation and
stores the result in a pixman_region16_t, but otherwise it serves the
same purpose.

Signed-off-by: Philipp Zabel <philipp.zabel@gmail.com>