Some recent painting-efficiency fix broke the inspector, which
accidentally depended on things getting repainted too often, and so
was failing to highlight things properly now. A simple queue_redraw()
fixes this, but while I was there, I decided to port the drawing hook
to JS as well, since all the necessary parts of cogl work fine from
JS.
https://bugzilla.gnome.org/show_bug.cgi?id=642058