Bug#921323: Acknowledgement (gdm3 ignores external monitors)

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I did some further testing and with Debian Stable (Stretch) everything
is working fine and also the monitors.xml gets interpreted properly.

Also with Testing (Buster) the monitors.xml seems to be interpreted
correctly (laptop screen as only connected monitor), tested with
rotated login screen to the right.

So it seems that gdm3 has problems to recognize the multi monitor setup
or if lid is closed and one external monitor is connected, the external
one.

I also disabled in logind.conf all lid close actions so that the laptop
did not suspend, but even then the login screen does not appear on the
external monitors.