Change resolution with "Mirror displays" checked, extended mode will be trimmed

Steps to reproduce:

1. Boot up the device, then plug in HDMI/VGA port
2. Check "Mirror displays" in Display of System Settings
3. Set the screen to lower resolution (e.g. 800x600) and apply change
4. Toggling Fn+F4 for a cycle, can't switch to extended mode

Expected Result:
Display modes should be "Mirror mode" -> "Extended mode" -> "Internal only mode" -> "External only mode"

Actual Result:
Display modes are "Mirror mode" -> "Mirror mode" -> "Internal only mode" -> "External only mode"

Note:
If unchecked "Mirror displays" and apply again, g-s-d can get the Xinerama configuration.

Some error messages while running g-s-d in debug mode:

(gnome-settings-daemon:4025): GnomeDesktop-CRITICAL **: gnome_rr_output_info_get_geometry: assertion `GNOME_IS_RR_OUTPUT_INFO (self)' failed

(gnome-settings-daemon:4025): GnomeDesktop-CRITICAL **: gnome_rr_output_info_get_geometry: assertion `GNOME_IS_RR_OUTPUT_INFO (self)' failed

(gnome-settings-daemon:4025): GnomeDesktop-CRITICAL **: gnome_rr_config_set_clone: assertion `GNOME_IS_RR_CONFIG (self)' failed
=== xinerama setup Configuration ===
  none

Since the gnome_rr_output_info_get_geometry() failed in compare_output_position(), trim_rightmost_outputs_that_dont_fit_in_framebuffer() will remove the Xinerama configuration in make_xinerama_setup().