paned: Initialize clip properly
paned: Initialize clip properly

This was causing problems in the case when only one of the paned
children is visible - we would use uninitialized memory, leading
to invalide clip regions. Concretely, the signal tab in the inspector
would sometimes not render at all.

grubersjoe/adwaita,grubersjoe/adwaita,grubersjoe/adwaita,grubersjoe/adwaita,grubersjoe/adwaita,grubersjoe/adwaita,grubersjoe/adwaita,grubersjoe/adwaita