break down GdkEventExpose into distinct rectangles for canvas expose rather than drawing the entire region as a single rect
reduce number of points used to define x-fade curves
fix/adjust logic used to display automation lines
only queue an item redraw from Item::show() or Item::Hide() if the call changes the visibility status of the item (i.e. make no-ops really be no-ops)
tentative intermediate state for ArdourCanvas::Curve
"correct" curve drawing (no artifacts during redraw)

Interpolation errors still need addressing.