Instead of blindly hoping that `$INCLUDE` contains the parent directory
of `gsettings-desktop-schemas`.
Because `gsettings-desktop-schemas.pc` says:
```
Cflags: -I/SOME/DIRECTORY/gsettings-desktop-schemas
```
Which means to include the version that Meson has configured you need
to drop the directory prefix and only `#include <gdesktop-enums.h>`.
This fixes a build failure with local installs triggered by 775ec67a44
but it's also the right thing to do™.
https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1370
MetaBackgroundContent is a ClutterContent implementation
that can render a background to any attached actor. Right
now, it preserves all the properties and the rendering
model of MetaBackgroundActor.
https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1302