Revert a const ref on update texture

if mod is a reference to a class member a variable swap breaks.
We should find a way to keep this const ref if possible.
Added a comment about this in header

src/content_cao.h

@@ -200,7 +200,9 @@ public:
 
 	void updateTexturePos();
 
-	void updateTextures(const std::string &mod);
+	// std::string copy is mandatory as mod can be a class member and there is a swap
+	// on those class members
+	void updateTextures(std::string mod);
 
 	void updateAnimation();