Don't draw the background before starting the session.

Drawing the background on the root window causes the following issues:

- The settings daemon sets the DE resolution and that can break the aspect of the background
- Depending on configuration and distros, the background rendered by the greeter may or may not be the same as the users's.
- Some DEs such as Cinnamon render a login animation, which looks good on a black screen but not when a background is drawn.

Potential for regression:

Minimal DEs and WM should be tested to ensure the default state of the root window is OK.
We might have to clear that window or explicitly paint it black (or the background color chosen by the user).