hash implementation for Graphic objects, for set/map use

Change-Id: I58547ad90d1aba6b1734a547c8138d1476c4b813
Reviewed-on: https://gerrit.libreoffice.org/49072Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Tomaž Vajngerl <quikee@gmail.com>

include/vcl/graph.hxx

@@ -191,6 +191,11 @@ public:
 
     BitmapChecksum  GetChecksum() const;
 
+    SAL_DLLPRIVATE std::size_t getHash() const
+    {
+        return reinterpret_cast<std::size_t>(ImplGetImpGraphic());
+    }
+
 public:
 
     std::shared_ptr<GraphicReader>& GetContext();
@@ -227,6 +232,19 @@ public:
     static css::uno::Sequence<sal_Int8> getUnoTunnelId();
 };
 
+namespace std {
+
+template <>
+struct hash<Graphic>
+{
+    std::size_t operator()(Graphic const & rGraphic) const
+    {
+        return rGraphic.getHash();
+    }
+};
+
+} // end namespace std
+
 #endif // INCLUDED_VCL_GRAPH_HXX
 
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