Add wxGraphicsRenderer::CreateContextFromUnknownDC()

It seems wrong to have this function in wxGraphicsContext only and not
in wxGraphicsRenderer, as this makes it impossible to create a context
associated with a non-default renderer while it doesn't cost us anything
to allow doing this.

include/wx/graphics.h

@@ -876,6 +876,8 @@ public:
 #endif
 #endif
 
+    wxGraphicsContext* CreateContextFromUnknownDC(const wxDC& dc);
+
     virtual wxGraphicsContext * CreateContextFromNativeContext( void * context ) = 0;
 
     virtual wxGraphicsContext * CreateContextFromNativeWindow( void * window ) = 0;