Make disabling joints possible. (#519)

Author: shanecelis

src/dynamics/solver/joints/mod.rs

@@ -338,3 +338,10 @@ impl AngleLimit {
         None
     }
 }
+
+/// Disables the joint of the entity it is placed on.
+#[derive(Reflect, Clone, Copy, Component, Debug)]
+#[cfg_attr(feature = "serialize", derive(serde::Serialize, serde::Deserialize))]
+#[cfg_attr(feature = "serialize", reflect(Serialize, Deserialize))]
+#[reflect(Debug, Component)]
+pub struct JointDisabled;

src/dynamics/solver/xpbd/mod.rs

@@ -351,7 +351,7 @@ pub trait XpbdConstraint<const ENTITY_COUNT: usize>: MapEntities {
 pub fn solve_constraint<C: XpbdConstraint<ENTITY_COUNT> + Component, const ENTITY_COUNT: usize>(
     mut commands: Commands,
     mut bodies: Query<RigidBodyQuery>,
-    mut constraints: Query<&mut C, Without<RigidBody>>,
+    mut constraints: Query<&mut C, (Without<RigidBody>, Without<JointDisabled>)>,
     time: Res<Time>,
 ) {
     let delta_secs = time.delta_seconds_adjusted();