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.

common/base_screen.cpp

@@ -423,6 +423,18 @@ wxPoint BASE_SCREEN::GetCursorPosition( bool aOnGrid, wxRealPoint* aGridSize )
 }
 
 
+wxPoint BASE_SCREEN::GetCrossHairScreenPosition() const
+{
+    wxPoint pos = m_Curseur - m_DrawOrg;
+    double scalar = GetScalingFactor();
+
+    pos.x = wxRound( (double) pos.x * scalar );
+    pos.y = wxRound( (double) pos.y * scalar );
+
+    return pos;
+}
+
+
 /* free the undo and the redo lists
  */
 void BASE_SCREEN::ClearUndoRedoList()