"SnapTo" feature enabled in the curve editor + non periodic handling of the flat curve editor.

Here is the new modifiew key behaviour :

    * CONTROL while dragging a point :
      slow down 20x the point move (i.e. 1pixel of cursor move move the control point by 0.05px)

    * SHIFT while dragging a point :
      snap the point to the following elements, on a "use nearest solution" method :
          o Diagonal curve editor :
                + top bound,
                + bottom bound,
                + identity (diagonal) line,
                + same Y than previous point,
                + same Y than next point,
                + line made by the previous and next point ; this will help to create linear parts in the curve more easilly
          o Flat curve editor (when moving a point) :
                + top bound,
                + bottom bound,
                + identity (horizontal) line,
                + same Y than previous point (or last point if you move the first point),
                + same Y than next point (or first point if you move the last point
          o Flat curve editor (when moving a tangential handle) :
                + 0.0, 0.35, 0.5, 1.0 values
                  0.35 is the default handles value and does create an smooth "diagonal" transition between points.

rtgui/mycurve.h

@@ -70,12 +70,25 @@ class MyCurve : public Gtk::DrawingArea {
 		std::vector<Gdk::Point> upoint;
 		std::vector<Gdk::Point> lpoint;
 		bool buttonPressed;
-		enum SnapToType snapTo;
+		/*
+		 * snapToElmt must be interpreted like this:
+		 * -100     : no element (default)
+		 * -3       : maximum value
+		 * -2       : identity value
+		 * -1       : minimum value
+		 * [0;1000[ : control point that it's snapped to
+		 * >=1000   : moved control point which is snapped to to the line made by it neighbors
+		 */
+		int snapToElmt;
+		bool snapTo;
+		double snapToMinDist;
+		double snapToVal;
 		MyCurveIdleHelper* mcih;
 		enum ResizeState sized;
 
 	    virtual std::vector<double> get_vector (int veclen) = 0;
 		int getGraphMinSize() { return GRAPH_SIZE + RADIUS + 1; }
+		bool snapCoordinate(double testedVal, double realVal);
 
 	public:
 		MyCurve ();