replace Xvfb with Xvnc or Xdummy when using xpra

[Antoine Martin]

Xvfb is old and it is missing a number of useful X11 extensions, also the alternatives may offer better keyboard support?
The idea of using those as an alternative to Xvfb is not new either, see ​x11vnc faq-xvfb

Information about dummy driver is scarce, but there is some in ​XorgTesting

The easiest way (which seems to work ok) is to use ​Xdummy (GPLv2+)

One of the most important benefits is that these alternatives also support randr so we could envisage resizing the buffer to the size needed by the client currently connected rather than the default huge size we have at the moment.