gdk: Add GDK_DEBUG=nogl

This is mostly useful for fallback testing.

I suppose if people want finer grained GL ability testing, they can use
Mesa environment variables to tune things.
glarea: Display the error message when an error occurred

Simply center a PangoLayout with the message text.
widget-factory: Add some vertical examples

This adds both a vertical toolbar and vertically linked buttons.
widget-factory: Add a missing scrolled window