Bug#785782: False sighting (will re-close) - problem was actually an invisible primary display

[Tim Day]

OK it turned out the gmd3 login prompt dialog wasn't actually absent... the
issue was due to my install actually thinking the machine had two heads,
when it didn't (7700K i7; CPU's integrated graphics on an Asrock H270 Pro4
mobo, one monitor attached to VGA port... but the freshly installed system
seemed to think there was a monitor on one on the other ports too).  The
dialog was presumably on the invisible-to-me display.  On closer inspection
I could slide the mouse off the left hand side of the screen I could see.
I think lightdm appeared to work better as it has he login dialog follow
the mouse from screen to screen (maybe gdm3 would be improved by such
behaviour; couldn't see anything in its config to select anything like that
though).

Anyway, the two screens issue became apparent when I looked closer at the
desktop lightdm was logging me into and I was obviously looking at the
secondary screen of a primary+secondary pair.  Reconfiguring displays
(usual right click on desktop) to be just a single primary on the desktop I
do have worked good.

The trick was then to copy the logged in user's .config/monitors.xml over
to where gdm3 could use it:

sudo cp .config/monitors.xml /var/lib/gdm3/.config/
sudo chown Debian-gdm:Debian-gdm /var/lib/gdm3/.config/monitors.xml

And after a dpkg-reconfigure gdm3 and selecting it as the login manager,
and a system restart, I'm seeing the expected gdm3 login prompt.  Phew!

(Makes me wonder if the Thinkpad I also ran into this on had the same
problem.)

Will send a separate control email to re-close this issue.
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