The attached changes fix flickering xwidgets by moving xwidget display
into a separate X window, and also enable the scrolling optimization for
xwidgets.

Someone with access to a Mac will have to do the changes necessary for
xwidget scrolling to work correctly on macOS, as they don't work on
GNUstep in the first place.

However, event passthrough doesn't work yet, and will require some
thought.  But that would be a fantastic opportunity to clean up the
current xwidget event handling mess (and perhaps even define a mechanism
for Lisp code to send events into xwidgets, which would make integrating
them into the existing Emacs event model much easier.)  Any thoughts?

Thanks.