Allow mutter to run nested in bigger resolution/scale than 800x600x1
Please allow mutter to run nested in bigger resolution/scale. It's needed for Firefox testing - Mozilla reftests needs 1280x1024 screen resolution for correct work. Also please provide screen scale (2x) to test Firefox on scaled displays.
Note that KWim support both (custom screen resolutions and scale) in nested mode so we may use it in Mozilla test suite until mutter provides the same.