Fix and enable optimized grid drawing and other minor changes.

* Fix optimized bitmap grid drawing method.
* Enable optimized grid drawing method on Windows and Linux.
* Create a helper class for resetting and restoring device context scale
  and origin settings for blitting purposes.
* Use wxLogTrace instead of wxLogDebug for coordinate dumping in
  drawpanel.cpp.  See comments for more information on enabling coordinate
  tracing.
* Add flag to allow hiding the drawing cross hair.  Hide cross hair by
  default on OSX.
* Move get cross hair device position code from draw panel object to base
  screen object.
* Remove redundant parent member variable from draw panel object by
  overriding wxWindow::GetParent() method.

include/class_base_screen.h

@@ -244,7 +244,7 @@ public:
      * @return the the current scale used to draw items on screen
      * draw coordinates are user coordinates * GetScalingFactor( )
      */
-    double  GetScalingFactor()
+    double  GetScalingFactor() const
     {
         return (double) m_ZoomScalar / (double) GetZoom();
     }
@@ -365,6 +365,13 @@ public:
      */
     wxPoint GetCursorPosition( bool aOnGrid, wxRealPoint* aGridSize = NULL );
 
+    /**
+     * Function GetCursorScreenPosition
+     * returns the cross hair position in device (display) units.b
+     * @return The current cross hair position.
+     */
+    wxPoint GetCrossHairScreenPosition() const;
+
     /**
      * Function GetNearestGridPosition
      * returns the nearest \a aGridSize location to \a aPosition.