Add the ability to respawn and improve controller port mapping (#72)

Reviewed-on: #72
Co-authored-by: Álex Román Núñez <eirexe@eirteam.moe>

src/stage/manifest.rs

@@ -91,7 +91,9 @@ pub struct StageManifest {
 
     #[reflect(ignore)]
     pub spawn_points: Vec<FGVec2>,
-    pub neutral_spawn_point_sets: Vec<Vec<usize>>
+    pub neutral_spawn_point_sets: Vec<Vec<usize>>,
+    #[reflect(ignore)]
+    pub respawn_points: Vec<FGVec2>,
 }
 
 #[derive(Resource)]
@@ -230,6 +232,10 @@ impl StageManifest {
             return Err("no spawn points are configured".to_owned());
         }
 
+        if self.respawn_points.is_empty() {
+            return Err("no respawn points are configured".to_owned());
+        }
+
         if self.neutral_spawn_point_sets.is_empty() {
             return Err("no neutral spawn layouts are configured".to_owned());
         }