Spread bitmap store hash table elements

XOR of two sequential ints makes for an inefficient hash table.  The
hash_combine function is intended for this purpose

common/bitmap_store.cpp

@@ -28,6 +28,7 @@
 #include <bitmaps.h>
 #include <bitmap_store.h>
 #include <bitmaps/bitmap_info.h>
+#include <hash_eda.h>
 #include <kiplatform/ui.h>
 #include <paths.h>
 #include <pgm_base.h>
@@ -93,7 +94,10 @@ static const wxString IMAGE_ARCHIVE = wxT( "images.tar.gz" );
 
 size_t std::hash<std::pair<BITMAPS, int>>::operator()( const std::pair<BITMAPS, int>& aPair ) const
 {
-    return std::hash<int>()( static_cast<int>( aPair.first ) ^ std::hash<int>()( aPair.second ) );
+    std::size_t seed = 0xa82de1c0;
+    hash_combine( seed, static_cast<size_t>( aPair.first ), static_cast<size_t>( aPair.second ) );
+
+    return seed;
 }