Make wxSVGFileDC::GetClippingBox() actually work

wxSVGFileDCImpl class uses the default, i.e. inherited from wxDCImpl,
implementation of this method, but for it to work, the clipping box
coordinates stored in wxDCImpl need to be updated when the clipping
region changes or is destroyed and this wasn't done before.

Fix this now and add a unit test verifying that this indeed works.

src/common/dcsvg.cpp

@@ -934,6 +934,9 @@ void wxSVGFileDCImpl::DoSetClippingRegion(int x, int y, int width, int height)
 
     m_clipUniqueId++;
     m_clipNestingLevel++;
+
+    // Update the base class m_clip[XY][12] fields too.
+    wxDCImpl::DoSetClippingRegion(x, y, width, height);
 }
 
 void wxSVGFileDCImpl::DestroyClippingRegion()
@@ -958,6 +961,9 @@ void wxSVGFileDCImpl::DestroyClippingRegion()
     DoStartNewGraphics();
 
     m_clipUniqueId = 0;
+
+    // Also update the base class clipping region information.
+    wxDCImpl::DestroyClippingRegion();
 }
 
 void wxSVGFileDCImpl::DoGetTextExtent(const wxString& string, wxCoord *w, wxCoord *h, wxCoord *descent, wxCoord *externalLeading, const wxFont *font) const