Store clipping box coordinates in device units for generic wxDC

Clipping box coordinates should be stored internally in device units
to be independent on changes of logical coordinates made between calls
to wxDCImpl::DoSetClippingRegion().
These stored coordinates should be converted on demand to the current
logical units on call to wxDCImpl::DoGetClippingRect().

include/wx/dc.h

@@ -742,7 +742,8 @@ protected:
 
     // bounding and clipping boxes
     wxCoord m_minX, m_minY, m_maxX, m_maxY; // Bounding box is stored in device units.
-    wxCoord m_clipX1, m_clipY1, m_clipX2, m_clipY2;  // Clipping box is stored in logical units.
+    wxCoord m_clipX1, m_clipY1, m_clipX2, m_clipY2;  // Clipping box is stored in device units.
+                                                     // Derived classes may store it in logical units.
 
     wxRasterOperationMode m_logicalFunction;
     int m_backgroundMode;