worker: move delta computation
worker: move delta computation

Move delta computation from red_add_drawable to red_current_add_with_shadow.

Acked-by: Fabiano Fidêncio <2931bae082ef9bc04523ba759c03a4778316d48f@redhat.com>

SPICE/spice,fgouget/spice,fgouget/spice,SPICE/spice,SPICE/spice,fgouget/spice,fgouget/spice