DnD target outline clipping issues

[Alexander Larsson <alexl redhat com>]

I just debugged this client-side-window failure:

http://tbzatek.fedorapeople.org/csw/Screenshot-Evolution-drag.png

Basically, with client side windows you get (got) the DnD outline when
dragging a dnd over the treeview. The weird thing about this bug is that
you don't really expect it to *not* happen for the native window case.

Turns out that the client-side-window bug was that non-double-buffered
cairo rendering did not clip against child client-side windows, whereas
cairo drawing does clip against native child windows. This has now been
fixed in the client-side-windows branch.

However, is it really right that drag outlines are never shown on
treeviews? We do draw it, but the child window of the treeview widget
causes it to clipped. Maybe we need to fix gtk_drag_highlight() /
gtk_drag_highlight_expose()?

Opinions?