Use wxDCClipper in wxHtmlDCRenderer code

Use wxDCClipper instead of manually calling DestroyClippingRegion().
This is simpler and safer and also potentially more correct as
wxDCClipper restores the previous clipping region, if any, unlike the
old code -- even though this probably doesn't matter here as the
printing DC seems unlikely to be clipped.

src/html/htmprint.cpp

@@ -161,12 +161,11 @@ int wxHtmlDCRenderer::Render(int x, int y,
         wxDefaultHtmlRenderingStyle rstyle;
         rinfo.SetStyle(&rstyle);
         m_DC->SetBrush(*wxWHITE_BRUSH);
-        m_DC->SetClippingRegion(x, y, m_Width, hght);
+        wxDCClipper clip(*m_DC, x, y, m_Width, hght);
         m_Cells->Draw(*m_DC,
                       x, (y - from),
                       y, y + hght,
                       rinfo);
-        m_DC->DestroyClippingRegion();
     }
 
     if (pbreak < m_Cells->GetHeight()) return pbreak;