added wxDC::ResetBoundingBox(), documented it and CalcBoundingBox() too

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/branches/WX_2_2_BRANCH@7496 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

include/wx/dc.h

@@ -48,9 +48,8 @@ public:
     {
         m_clipping = FALSE;
         m_ok = TRUE;
-        m_isBBoxValid = FALSE;
 
-        m_minX = m_minY = m_maxX = m_maxY = 0;
+        ResetBoundingBox();
 
         m_signX = m_signY = 1;
 
@@ -419,6 +418,13 @@ public:
       }
     }
 
+    void ResetBoundingBox()
+    {
+        m_isBBoxValid = FALSE;
+
+        m_minX = m_maxX = m_minY = m_maxY = 0;
+    }
+
     // Get the final bounding box of the PostScript or Metafile picture.
     wxCoord MinX() const { return m_minX; }
     wxCoord MaxX() const { return m_maxX; }