added constructor for external surfaces

Image.cpp

@@ -35,3 +35,7 @@ Image::Image(const std::string& path) {
     if (!loaded) throw std::runtime_error(IMG_GetError());
     _s.reset(loaded);
 }
+
+Image::Image(SDL_Surface* surface) {
+    _s.reset(surface);
+}

Image.hpp

@@ -19,6 +19,7 @@ namespace sc {
             public:
                 Image(int w, int h);
                 Image(const std::string& path);
+                Image(SDL_Surface* surface);
 
                 SDL_Surface* surface() const { return _s.get(); }
                 int width() const { return _s.get()->w; }