GerbView rendering bug fix and other minor improvements.

* Fix rendering bug in GerbView caused by wxDC scaling change caused by
  setting the wxDC clipping region.
* Eliminate the need to have optional background erasing as the change
  above fixed that problem as well.
* Default cursor handling improvements.

common/drawframe.cpp

@@ -388,10 +388,9 @@ void EDA_DRAW_FRAME::SetToolID( int aId, int aCursor, const wxString& aToolMsg )
     // Keep default cursor in toolbars
     SetCursor( wxNullCursor );
 
-    // Change Cursor in DrawPanel only
-    if( DrawPanel )
+    // Change DrawPanel cursor if requested.
+    if( DrawPanel && aCursor >= 0 )
     {
-        DrawPanel->m_PanelDefaultCursor = aCursor;
         DrawPanel->SetCursor( aCursor );
     }