make it easier to set initial resolution using --resize-display=geometry

[Antoine Martin]

XPRA_DEFAULT_VFB_RESOLUTION was added in #1132.

We could just support: --resize-display=1600x1200.

Also make it possible to honour the --pixel-depth with Xvfb.

[Antoine Martin]

Done in r26423. (r26424 for man page)

Examples:

xpra start-desktop --resize-display=1280x1024 --pixel-depth=30

This will also work for Xvfb, which will now start with a 30-bit display.