title: Bezier curve through three points
body: <p>I have read similar topics in order to find solution, but with no success.
  What I'm trying to do is make the tool same as can be found in CorelDraw, named "Pen Tool". I did it by connecting Bezier cubic curves, but still missing one feature, which is dragging curve (not control point) in order to edit its shape.</p>
  
  <p>I can successfully determine the "t" parameter on the curve where dragging should begin, but don't know how to recalculate control points of that curve.</p>
  
  <p>Here I want to higlight some things related to CorelDraw''s PenTool behaviour that may be used as constaints. I've noticed that when dragging curve strictly vertically, or horizontally, control points of that Bezier curve behave accordingly, i.e. they move on their verticals, or horizontals, respectively.</p>
  
  <p>So, how can I recalculate positions of control points while curve dragging?</p>