Fix build with Visual Studio

I also fixed a compiler warning.

src/image_modifications.cpp

@@ -520,7 +520,7 @@ std::pair<int,int> scale_into_modification::calculate_size(const surface& src) c
 
 	long double ratio = std::min(w / old_w, h / old_h);
 
-	return {old_w * ratio, old_h * ratio};
+	return {static_cast<int>(old_w * ratio), static_cast<int>(old_h * ratio)};
 }
 
 surface xbrz_modification::operator()(const surface& src) const

src/image_modifications.hpp

@@ -51,7 +51,7 @@ public:
 
 private:
 	/// Map from a mod's priority() to the mods having that priority.
-	typedef std::map<int, std::vector<std::unique_ptr<modification>>, std::greater<int>> map_type;
+	typedef std::map<int, std::vector<std::shared_ptr<modification>>, std::greater<int>> map_type;
 	/// Map from a mod's priority() to the mods having that priority.
 	map_type priorities_;
 };