xrandr: the best supported resolution

[rayrapetyan]

Not all resolutions are supported in different environments. We need to find the best supported resolution on a given system. E.g. for:

- screen:
    resolution: 1280,768

when:
xrandr returns:

   1920x1200     59.95*+
   1920x1080     60.00  
   1680x1050     59.95  
   1600x1200     60.00  
   1280x1024     60.02  
   1280x960      60.00  
   1024x768      60.00  
   800x600       60.32  
   640x480       59.94

1280x960 should be used.