Keyframe::GetPoint() returns a constant reference now

Returning a non constant reference is not possible; this allows
outside code to invalidate internal invariants (sorted order of
points) by modifying the returned point.  To make updates the current
best approach is to remove the point by index, and then add it again.

include/KeyFrame.h

@@ -125,7 +125,7 @@ namespace openshot {
 		double GetDelta(int64_t index);
 
 		/// Get a point at a specific index
-		Point& GetPoint(int64_t index);
+		Point const & GetPoint(int64_t index);
 
 		/// Get current point (or closest point to the right) from the X coordinate (i.e. the frame number)
 		Point GetClosestPoint(Point p);

src/KeyFrame.cpp

@@ -449,7 +449,7 @@ double Keyframe::GetDelta(int64_t index)
 }
 
 // Get a point at a specific index
-Point& Keyframe::GetPoint(int64_t index) {
+Point const & Keyframe::GetPoint(int64_t index) {
 	// Is index a valid point?
 	if (index >= 0 && index < Points.size())
 		return Points[index];