kill() allows now paint with defined colors over rendered objects
without restore of background. It's similar to paintBackgroundBoxRel()
known from CFrameBuffer, but with possiblity to define color,
default color is 0 like before (empty background).
Items with parent bindings use the background color of its parent
as default, but can also be ignored.
This function can be useful before repaint of items and/or,
if required, to have a clean background inside item containers.
Thilo Graf
6a2e701a7a