Begin adding entities to editor

src/rpg/entity/entity.c

@@ -188,4 +188,13 @@ entity_t * entityGetAt(const worldpos_t position) {
   } while(++ent, ent < &ENTITIES[ENTITY_COUNT]);
 
   return NULL;
+}
+
+uint8_t entityGetAvailable() {
+  entity_t *ent = ENTITIES;
+  do {
+    if(ent->type == ENTITY_TYPE_NULL) return ent - ENTITIES;
+  } while(++ent, ent < &ENTITIES[ENTITY_COUNT]);
+
+  return 0xFF;
 }