Automatically flush window when doing non-double-buffered expose

Automatically flush window when doing non-double-buffered expose

Applications using non-double-buffered drawing using cairo (e.g. Abiword)
can draw directly to the window using cairo and thus manage to avoid
the automatic flushing of outstanding moves. This can cause redraw
inconsistencies like bug 593507.

We fix this by always flushing when exposing non-double-buffered widgets.