Fix: Acceleration needs to be reset to 0

The physics integrator is real scuffed. Nothing has mass, and no forces
are applied. Instead, objects affect each other's acceleration. It acts
functionally like a force accumulator where everything has a mass of 1.

Under this model, it must be cleared each frame so that it can be
re-accumulated during the next physics calculation cycle.

This bug was spotted because of anomolous behavior of the velocity
pointer. Nearly stationary boids would have a rather large (especially
after the coefficient reduction earlier) velocity. I noticed this
earlier when building the debug tool and thought it was an error in
the calculations. There might still be bugs, I still need to test, but
that particular one seems to have been a position integrator error.

src/birdoids_plugin.rs

@@ -143,11 +143,12 @@ fn turn_if_edge(
 }
 
 fn apply_velocity(
-    mut query: Query<(&mut Transform, &Velocity, &Acceleration)>,
+    mut query: Query<(&mut Transform, &Velocity, &mut Acceleration)>,
     time: Res<Time>
 ) {
-    for (mut transform, velocity, acceleration) in &mut query {
+    for (mut transform, velocity, mut acceleration) in &mut query {
         let delta_v = **acceleration * time.delta_seconds();
+        **acceleration = Vec3::ZERO;
         let delta_position = (**velocity + delta_v) * time.delta_seconds();
         transform.translation += delta_position;
     }