Get screen size from CRTC

On virtual desktops depending on the driver xrandr doesn't always return
the current screen/viewport size. CRTCs though are always correct and
also provide the offset in the virtual space.
Add --x-output command line option

The option accepts either a name or XID of X output.
Use the X output from Calibrator to get a specific CRTC

The string is parsed to XID or output name and with it the screen
resources are queried for the CRTC to which the output is connected.
If no X output is specified the first valid CRTC for the screen is used.
Optionally accept x and y offsets for --geometry
Merge branch 'virtual-desktop' into test2