Canvas: fix position for drag events and multiclick handling.

- Calculate local/client position for every drag event
   instead of just reporting the position of the initial
   mousedown event.
 - Only pressing the same button multiple times increases
   the mouse clickcount.

simgear/canvas/elements/CanvasElement.cxx

@@ -303,6 +303,18 @@ namespace canvas
     return _transform.get();
   }
 
+  //----------------------------------------------------------------------------
+  osg::Vec2f Element::posToLocal(const osg::Vec2f& pos) const
+  {
+    getMatrix();
+    const osg::Matrix& m = _transform->getInverseMatrix();
+    return osg::Vec2f
+    (
+      m(0, 0) * pos[0] + m(1, 0) * pos[1] + m(3, 0),
+      m(0, 1) * pos[0] + m(1, 1) * pos[1] + m(3, 1)
+    );
+  }
+
   //----------------------------------------------------------------------------
   void Element::childAdded(SGPropertyNode* parent, SGPropertyNode* child)
   {

simgear/canvas/elements/CanvasElement.hxx

@@ -129,6 +129,11 @@ namespace canvas
       osg::MatrixTransform* getMatrixTransform();
       osg::MatrixTransform const* getMatrixTransform() const;
 
+      /**
+       * Transform position to local coordinages.
+       */
+      osg::Vec2f posToLocal(const osg::Vec2f& pos) const;
+
       virtual void childAdded( SGPropertyNode * parent,
                                SGPropertyNode * child );
       virtual void childRemoved( SGPropertyNode * parent,