wxOSX: Implement function to retrieve pattern image for non-solid colour

Under wxOSX non-solid wxColour with NSColor backend is represented by
pattern image and it would be useful to have ability to get it e.g. to help
dealing with non-solid wxColours by frameworks not supporting such colours,
like e.g. Cairo.

include/wx/osx/core/colour.h

@@ -68,6 +68,7 @@ public:
     // This ctor does not take ownership of the color.
     explicit wxColour(WX_NSColor color);
     WX_NSColor OSXGetNSColor() const;
+    WX_NSImage OSXGetNSPatternImage() const;
 #endif
 
 protected :
@@ -102,6 +103,7 @@ public:
 
 #if wxOSX_USE_COCOA
     virtual WX_NSColor GetNSColor() const;
+    virtual WX_NSImage GetNSPatternImage() const;
 #endif
 };