Fix clear last entity alive

In the `Entities` vec, an entity is considered alive if its index is equal to its position in the `Entities` vec
`clear` was always setting the last entity index to its position in the vec
This made `clear` always leaving an entity alive, without component

src/entities/mod.rs

@@ -392,7 +392,13 @@ impl Storage for Entities {
             return;
         }
 
-        let mut last_alive = self.data.len() as u64 - 1;
+        // the first value can be anything but self.data.len() - 1
+        // otherwise we would set data[len - 1].index to len - 1 and not delete it
+        let mut last_alive = if self.data.len() as u64 == EntityId::max_index() {
+            0
+        } else {
+            EntityId::max_index()
+        };
         for (i, id) in self.data.iter_mut().enumerate().rev() {
             let target = last_alive;
             let id_before_bump = *id;

tests/clear.rs

@@ -18,15 +18,23 @@ fn no_pack() {
 
     let (mut entities, mut u32s) = world.borrow::<(EntitiesViewMut, ViewMut<U32>)>().unwrap();
 
-    entities.add_entity(&mut u32s, U32(0));
-    entities.add_entity(&mut u32s, U32(1));
-    entities.add_entity(&mut u32s, U32(2));
+    let e0 = entities.add_entity(&mut u32s, U32(0));
+    let e1 = entities.add_entity(&mut u32s, U32(1));
+    let e2 = entities.add_entity(&mut u32s, U32(2));
 
     drop((entities, u32s));
     world.borrow::<AllStoragesViewMut>().unwrap().clear();
 
     let (mut entities, mut u32s) = world.borrow::<(EntitiesViewMut, ViewMut<U32>)>().unwrap();
 
+    assert!(u32s.get(e0).is_err());
+    assert!(u32s.get(e1).is_err());
+    assert!(u32s.get(e2).is_err());
+    assert!(!entities.is_alive(e0));
+    assert!(!entities.is_alive(e1));
+    assert!(!entities.is_alive(e2));
+    assert_eq!(entities.iter().count(), 0);
+
     assert_eq!(u32s.len(), 0);
     let entity0 = entities.add_entity(&mut u32s, U32(3));
     let entity1 = entities.add_entity(&mut u32s, U32(4));