Split from #1229. This isn't supported by GTK, because as per How can I lock the cursor to the inside of a window on Mac OS X?, there is no API to do this on osx (insert marketing blurb about how this is in fact an innovative feature). We would have to do this by hand when we receive motion events and move the pointer back where we want it. Yuk.
this ticket has been moved to: https://github.com/Xpra-org/xpra/issues/1326