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.

tests/graphics/clippingbox.cpp

@@ -21,7 +21,9 @@
 #if wxUSE_GRAPHICS_CONTEXT
 #include "wx/dcgraph.h"
 #endif // wxUSE_GRAPHICS_CONTEXT
+#include "wx/dcsvg.h"
 
+#include "testfile.h"
 
 // ----------------------------------------------------------------------------
 // test class
@@ -2122,3 +2124,29 @@ void ClippingBoxTestCaseGCBase::RegionsAndPushPopState()
 }
 
 #endif // wxUSE_GRAPHICS_CONTEXT
+
+#if wxUSE_SVG
+
+// We can't reuse the existing tests for wxSVGFileDC as we can't check its
+// output, but we can still at least check the behaviour of GetClippingBox().
+TEST_CASE("ClippingBoxTestCaseSVGDC", "[clip][svgdc]")
+{
+    TestFile tf;
+    wxSVGFileDC dc(tf.GetName(), s_dcSize.x, s_dcSize.y);
+
+    wxRect rect;
+    dc.GetClippingBox(rect);
+    CHECK( rect == wxRect(s_dcSize) );
+
+    const wxRect rectClip(10, 20, 80, 75);
+    dc.SetClippingRegion(rectClip);
+
+    dc.GetClippingBox(rect);
+    CHECK( rect == rectClip );
+
+    dc.DestroyClippingRegion();
+    dc.GetClippingBox(rect);
+    CHECK( rect == wxRect(s_dcSize) );
+}
+
+#endif // wxUSE_SVG