testgtk/Alpha Window: fix the drawn area when we are drawing decorations
testgtk/Alpha Window: fix the drawn area when we are drawing decorations

A GtkWindow's allocation includes the titlebar, borders, and shadows; we
only want to draw our custom alpha content over the child allocation of
the GtkWindow.

https://bugzilla.gnome.org/show_bug.cgi?id=756886

grubersjoe/adwaita,grubersjoe/adwaita,alexlarsson/gtk,alexlarsson/gtk,grubersjoe/adwaita,alexlarsson/gtk,alexlarsson/gtk,alexlarsson/gtk,alexlarsson/gtk,alexlarsson/gtk,alexlarsson/gtk,grubersjoe/adwaita,grubersjoe/adwaita,grubersjoe/adwaita,grubersjoe/adwaita,grubersjoe/adwaita