Add wxGraphicsContext::CreateFromUnknownDC()

A convenience helper for writing generic code that may operate on
different kinds of DCs, all supported by wxGraphicsContext, but without
knowing its specific type.

interface/wx/graphics.h

@@ -438,6 +438,21 @@ public:
     */
     static wxGraphicsContext* Create(const wxEnhMetaFileDC& metaFileDC);
 
+    /**
+        Creates a wxGraphicsContext from a DC of unknown specific type.
+
+        Creates a wxGraphicsContext if @a dc is a supported type (i.e. has a
+        corresponding Create() method, e.g. wxWindowDC or wxMemoryDC).
+        Returns @NULL if the DC is unsupported.
+
+        This method is only useful as a helper in generic code that operates
+        with wxDC and doesn't known its exact type. Use Create() instead if
+        you know that the DC is e.g. wxWindowDC.
+
+        @since 3.1.1
+     */
+    static wxGraphicsContext* CreateFromUnknownDC(wxDC& dc);
+
     /**
         Creates a wxGraphicsContext associated with a wxImage.