display: Set useful values for MIN_DISPLAY_{WIDTH, HEIGHT}
    
    Nowadays the value for MIN_DISPLAY_{WIDTH,HEIGHT} is 50. This arbitrary
    value doesn't bring any benefit, doesn't provide a useful size for a
    desktop to be usable and can actually trigger some undefined behavior
    when reaching resolutions that are lower than the ones provided by the
    video drivers (as in rhbz#1296878).
    
    In order to avoid these issues and provide a minimum resolution that can
    still be useful for our users, let's use the same values for minimum
    width and height used by the linux QXL drivers (320x200).
    
    This also requires us to adjust the minimum requested widget size when
    zoom is enabled so that we don't accidentally request a size smaller
    than the driver can support.
    
    Related: rhbz#1296878
    Signed-off-by: Fabiano Fidêncio <fidencio@redhat.com>