ScreenPool: add fake screens initially
Original commit message(s)
- ScreenPool: make sure to have screens added initially
So that fake screens at the gui starts are in m_fakeScreens.
Reason for the change
Should fix the X11 crash https://crash-reports.kde.org/organizations/kde/issues/3211/?project=9&query=is%3Aunresolved+issue.priority%3A%5Bhigh%2C+medium%5D&referrer=issue-stream&statsPeriod=30d&stream_index=4
Test plan
In X11, boot, go to sleep, plug-unplug a screen/dock. With assertions on (like on Neon).
Untested as of yet.