To make the CcBackgroundPaintable fully reusable, it has to know the
desired thumbnail size and the thumbnail factory. We add a width and
height property, and modify the thumbnail retrieval of CcBackgroundItem
to use the scale factor now.
This also allows us to greatly simplify the background sources, so that
they now only do one thing: provide CcBackgroundItems from different
sources.
We keep the fixed thumbnail height and width for the chooser, which
makes more sense now that it is in the chooser code instead of the
background source code.
In order to make the CcBackgroundPaintable more reusable, the way it
draws itself either light, dark, or both needs to be adjustable. We
therefore introduce CcBackgroundPaintFlags that indicate the desired
drawing behavior.
Turns out drawing the whole light wallpaper and then half of the dark
wallpaper on top of it is leading to a case of
https://gitlab.gnome.org/GNOME/gtk/-/issues/3840
Draw half of each instead, so they don't overlap.
