Shift top-left display to origin
    
    When using a custom fullscreen display configuration, it's possible to
    specify that e.g. a single screen should be fullscreen on client
    monitor #4. Since we send down absolute positions and disable alignment
    when all windows are in fullscreen, we can send configurations with a
    very large offset to the top-left corner. This could result in the guest
    trying to create a screen that was much larger than necessary. For
    example when sending a configuration of 1280x1024+4240+0, the guest
    would need to allocate a screen of size 5520x1024, which might fail if
    video memory was too low. To avoid this issue, we shift all displays
    so that the minimum X coordinate for all screens is at x=0, and the
    minimum y coordinate is at y=0.