It seems like many of the X11 operations can end up with no pixels
actually changing. So instead of discovering and adding workarounds for
each individually we'll just check very region added if it's empty.
This allows Xvnc to build with xorg-server 1.20.7, which requires OS
layers to implement a ddxInputThreadInit function when configured with
--enable-input-thread (the default).
relevant xorg-server commit: e3f26605d85d987da434640f52646d728f1fe919
When used with -fg we expect the startup script to remain running until
the session is over. This will not happen if the session command is put
in the background using &.
