Enabled SDL_image's support for PNG and TIFF files and improved the

AssetManager with a hashing function, a linked list structure to store
the hashes as well as a reference counter to support multiple users
sharing the same asset.

src/ObjectStore.c

@@ -49,7 +49,7 @@ ObjectID add_object_to_store(ObjectStore *os, void *object, EventHandlerPtr eh,
     printf("Failed to add object\n");
 #endif /* DEBUG_ENGINE */
 
-    return -1;
+    return INVALID_ID;
   }
 
   // TODO (Abdelrahman): Need to figure out a way to check that the object
@@ -78,11 +78,15 @@ ObjectID add_object_to_store(ObjectStore *os, void *object, EventHandlerPtr eh,
   return id;
 }
 
-void remove_object_from_store(ObjectStore *os, ObjectID id) {
-  if (!os || os->count == 0 || !(os->object_ptrs[id])) {
+void remove_object_from_store(ObjectStore *os, ObjectID *id_ptr) {
+  ObjectID id = *id_ptr;
+
+  if (!os || os->count == 0 || id == INVALID_ID || !(os->object_ptrs[id])) {
     return;
   }
 
+  *id_ptr = INVALID_ID;
+
   os->object_ptrs[id] = NULL;
   os->event_handlers[id] = NULL;
   os->update_funcs[id] = NULL;