Fix Time inconsistency after substepping loop (#294)

# Objective

There are currently two issues that cause `Time` to be inconsistent, even with a fixed timestep:

1. After the substepping loop, the `Time` resource is always set to `Time<Virtual>`. However, the `PhysicsSchedule` is still running. This means that systems running after substeps but before the end of the physics schedule will have a variable `Time`.
2. After the `PhysicsSchedule`, `Time` is set to `Time<Virtual>`. This means that if physics is run in `FixedUpdate`, the fixed timestep will be overwritten by `Time<Virtual>`.

## Solution

1. Set `Time` to `Time<Physics>` instead. The physics schedule will later set it to the default clock.
2. Set `Time` to the clock that was used before the `PhysicsSchedule`.

src/plugins/setup/mod.rs

@@ -217,6 +217,7 @@ fn run_physics_schedule(world: &mut World, mut is_first_run: Local<IsFirstRun>)
         let real_delta = world.resource::<Time<Real>>().delta();
         let old_delta = world.resource::<Time<Physics>>().delta();
         let is_paused = world.resource::<Time<Physics>>().is_paused();
+        let old_clock = world.resource::<Time>().as_generic();
         let physics_clock = world.resource_mut::<Time<Physics>>();
 
         // Get the scaled timestep delta time based on the timestep mode.
@@ -276,12 +277,12 @@ fn run_physics_schedule(world: &mut World, mut is_first_run: Local<IsFirstRun>)
                 .resource_mut::<Time<Physics>>()
                 .advance_by(Duration::ZERO);
         }
+
+        // Set generic `Time` resource back to the clock that was active before physics.
+        *world.resource_mut::<Time>() = old_clock;
     });
 
     is_first_run.0 = false;
-
-    // Set generic `Time` resource back to `Time<Virtual>`.
-    *world.resource_mut::<Time>() = world.resource::<Time<Virtual>>().as_generic();
 }
 
 /// Runs the [`SubstepSchedule`].
@@ -301,7 +302,9 @@ fn run_substep_schedule(world: &mut World) {
         }
     });
 
-    *world.resource_mut::<Time>() = world.resource::<Time<Virtual>>().as_generic();
+    // Set generic `Time` resource back to `Time<Physics>`.
+    // Later, it's set back to the default clock after the `PhysicsSchedule`.
+    *world.resource_mut::<Time>() = world.resource::<Time<Physics>>().as_generic();
 }
 
 /// Runs the [`PostProcessCollisions`] schedule.