Set initial mode as preferred.

If there is any resolution specified with -geometry or -screen parameters,
report this resolution as preferred one. That way desktop environments won't
change it immediately after start.
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Most desktop environments like to do some equivalent of "xrandr --output VNC-0 --auto" on start. Any user specified resolution is then replaced with the highest available one: 1920x1200
This patch marks user specified resolution (either a standard one, or new custom one) as preferred.