don't loose a redraw.

don't loose a redraw.

This more or less obsoletes the DisplaySuspender.
While RAII is nice, the DisplaySuspender cannot be
used from outside gtk2_ardour eg Mute/Solo signals
notifications from libardour (which don't originate
from the GUI).

The DisplaySuspender is still useful, because it
explicitly disables re-display and forces a single
expose at the end.