[LightDM] SwitchToGreeter vs. LogIn

[poma]

Guys, do not hesitate to comment if you can help.

In a nutshell, once spawn - SwitchToGreeter breaks LogIn.
So far this happens with GPU driven by Nouveau
and
LightDM >= 1.18.2

More specifically, breakage starts with:
https://bazaar.launchpad.net/~lightdm-team/lightdm/1.18/revision/2319

If you are going to test, whether it's working or not,
besides the test result, please also provide the following command output:

lspci -knn -d ::300


Test compilation:
http://goo.gl/Gm4ffO
iso/

How to test:
0. Boot System
1. Logi In
2. Switch User
3. Unlock
4. Log Out
5. Log In ?

Ref.
https://bugs.freedesktop.org/show_bug.cgi?id=96737