Fix the physics integrator

The velocity component never got updated. Instead, the system was only
calculating the current frame's deltaV. That instantaneous dV and any
velocity that *did* get assigned (bundle insertion, keyboard control)
would be added to the position.

Now forces are integrated into velocity, and velocity is integrated into
position. You know... like a real integration function.

src/birdoids/physics.rs

@@ -1,16 +1,23 @@
 use bevy::prelude::*;
 
-#[derive(Component, Deref, DerefMut)]
+#[derive(Component, Default, Deref, DerefMut)]
 pub struct Velocity(pub Vec3);
 
 #[derive(Component, Default, Deref, DerefMut, PartialEq, Debug)]
 pub struct Force(pub Vec3);
 
-pub fn apply_velocity(mut query: Query<(&mut Transform, &Velocity, &mut Force)>, time: Res<Time>) {
-    for (mut transform, velocity, mut acceleration) in &mut query {
-        let delta_v = **acceleration * time.delta_secs();
-        **acceleration = Vec3::ZERO;
-        let delta_position = (**velocity + delta_v) * time.delta_secs();
-        transform.translation += delta_position;
+pub fn apply_velocity(
+    mut query: Query<(&mut Transform, &mut Velocity, &mut Force)>,
+    time: Res<Time>,
+) {
+    for (mut transform, mut velocity, mut force) in &mut query {
+        // integrate forces into new velocity (assume mass of 1kg)
+        let delta_v = **force * time.delta_secs();
+        velocity.0 += delta_v;
+        force.0 = Vec3::ZERO; // clear force value
+
+        // integrate velocity into position
+        let delta_p = **velocity * time.delta_secs();
+        transform.translation += delta_p;
     }
 }