Fix look_to resulting in NaN rotations (bevyengine#7817)

Co-authored-by: Alice Cecile <alice.i.cecile@gmail.com>
Co-authored-by: Liam Gallagher <ljpgallagher@xtra.co.nz>

crates/bevy_transform/src/components/transform.rs

@@ -120,6 +120,11 @@ impl Transform {
 
     /// Returns this [`Transform`] with a new rotation so that [`Transform::forward`]
     /// points towards the `target` position and [`Transform::up`] points towards `up`.
+    ///
+    /// In some cases it's not possible to construct a rotation. Another axis will be picked in those cases:
+    /// * if `target` is the same as the transform translation, `Vec3::Z` is used instead
+    /// * if `up` is zero, `Vec3::Y` is used instead
+    /// * if the resulting forward direction is parallel with `up`, an orthogonal vector is used as the "right" direction
     #[inline]
     #[must_use]
     pub fn looking_at(mut self, target: Vec3, up: Vec3) -> Self {
@@ -129,6 +134,11 @@ impl Transform {
 
     /// Returns this [`Transform`] with a new rotation so that [`Transform::forward`]
     /// points in the given `direction` and [`Transform::up`] points towards `up`.
+    ///
+    /// In some cases it's not possible to construct a rotation. Another axis will be picked in those cases:
+    /// * if `direction` is zero, `Vec3::Z` is used instead
+    /// * if `up` is zero, `Vec3::Y` is used instead
+    /// * if `direction` is parallel with `up`, an orthogonal vector is used as the "right" direction
     #[inline]
     #[must_use]
     pub fn looking_to(mut self, direction: Vec3, up: Vec3) -> Self {
@@ -325,17 +335,31 @@ impl Transform {
 
     /// Rotates this [`Transform`] so that [`Transform::forward`] points towards the `target` position,
     /// and [`Transform::up`] points towards `up`.
+    ///
+    /// In some cases it's not possible to construct a rotation. Another axis will be picked in those cases:
+    /// * if `target` is the same as the transtorm translation, `Vec3::Z` is used instead
+    /// * if `up` is zero, `Vec3::Y` is used instead
+    /// * if the resulting forward direction is parallel with `up`, an orthogonal vector is used as the "right" direction
     #[inline]
     pub fn look_at(&mut self, target: Vec3, up: Vec3) {
         self.look_to(target - self.translation, up);
     }
 
     /// Rotates this [`Transform`] so that [`Transform::forward`] points in the given `direction`
     /// and [`Transform::up`] points towards `up`.
+    ///
+    /// In some cases it's not possible to construct a rotation. Another axis will be picked in those cases:
+    /// * if `direction` is zero, `Vec3::Z` is used instead
+    /// * if `up` is zero, `Vec3::Y` is used instead
+    /// * if `direction` is parallel with `up`, an orthogonal vector is used as the "right" direction
     #[inline]
     pub fn look_to(&mut self, direction: Vec3, up: Vec3) {
-        let forward = -direction.normalize();
-        let right = up.cross(forward).normalize();
+        let forward = -direction.try_normalize().unwrap_or(Vec3::Z);
+        let up = up.try_normalize().unwrap_or(Vec3::Y);
+        let right = up
+            .cross(forward)
+            .try_normalize()
+            .unwrap_or_else(|| up.any_orthonormal_vector());
         let up = forward.cross(right);
         self.rotation = Quat::from_mat3(&Mat3::from_cols(right, up, forward));
     }