Various changes intended to improve GUI responsiveness wrt drawing  ops.

	* don't redraw the summary incessantly while loading session.
	* use a background ImageSurface for summary containing track lines and regions.
	Composite with 'live' playhead, view rect etc. using cairo_push/pop_group
	* reduce use of cairo_stroke () when drawing waveforms (should be a visual no-op)
	* don't queue a draw for canvas items that are off window or !visible.