Xpra: Ticket #993: fully initialize the xvfb before start / start-child

This will fix a number of issues related to dpi and cursors, made more apparent by the scaling work (#976) and will help with other issues: #476, #228 and probably others. Similar to #513, #438, #369. We can then set sensible Xrdb defaults.

Sun, 04 Oct 2015 09:28:40 GMT - Antoine Martin: status, description changed; resolution set

Done in r10729.

Too big to backport, problem is that cursor scaling is a bit broken in older branches... but fixing it would require this change.

Sat, 23 Jan 2021 05:11:52 GMT - migration script:

this ticket has been moved to: https://github.com/Xpra-org/xpra/issues/993