Add PreRender signal to the canvas

Emitted by the canvas immediately before rendering.

libs/canvas/canvas.cc

@@ -101,6 +101,8 @@ Canvas::zoomed ()
 void
 Canvas::render (Rect const & area, Cairo::RefPtr<Cairo::Context> const & context) const
 {
+	PreRender (); // emit signal
+
 #ifdef CANVAS_DEBUG
 	if (DEBUG_ENABLED(PBD::DEBUG::CanvasRender)) {
 		cerr << this << " RENDER: " << area << endl;

libs/canvas/canvas/canvas.h

@@ -135,6 +135,8 @@ public:
 	*/
 	sigc::signal<void,Duple const&> MouseMotion;
 
+	sigc::signal<void> PreRender;
+
 	/** Ensures that the position given by @param winpos (in window
 	    coordinates) is within the current window area, possibly reduced by
 	    @param border.