Implemented Clear in wxSVGFileDC.

Draw a rectangle with the background colour and a transparent pen.
Closes #15788.

interface/wx/dcsvg.h

@@ -60,7 +60,7 @@ public:
     void EndPage();
 
     /**
-        This makes no sense in wxSVGFileDC and does nothing.
+        Draws a rectangle the size of the SVG using the wxDC::SetBackground() brush.
     */
     void Clear();