Fix look_to variable naming

[lewiszlw]

Objective

• If I understand correctly, forward points in direction, so the negative of direction should be back.

Migration Guide

• Transform::look_to method changed default value of direction.try_normalize() from Vec3::Z to Vec3::NEG_Z

[nicopap]

In rust the minus sign - has lower priority than method call, so it's actually -Z. Here is how it looks like with parenthesis:

        let back = -(  direction.try_normalize().unwrap_or(Vec3::Z)  );

I still suggest a change though. It's a bit clearer:

        let back = direction.try_normalize().map_or(-Vec3::Z, |d| -d);

The change from forward to back seems correct though.

[lewiszlw]

Why default value should be -Vec3::Z? According to my understanding, -Vec3::Z should be forward. @nicopap

[nicopap]

-Vec3::Z is to keep the same behavior as in 0.10.1 Any change to this value is a breaking change, it also needs to update the doc to reflect this change.

I think it's OK (but not important) to change it to -Vec3::NEG_Z (I would keep the - for clarity), but you need then to mark this PR as breaking (and update the doc)

[lewiszlw]

Added migration guide! Another question, I think unwrap_or might have better readability than map_or, at least for me.

[nicopap]

Keep it as unwrap_or if you think it's more readable. I think it's good enough.

I can't comment on it, but at line 354 there is this:

    /// * if `up` is zero, `Vec3::Y` is used instead

This needs to be replaced by

    /// * if `up` is zero, `Vec3::NEG_Y` is used instead

[lewiszlw]

You mean

if `direction` is zero, `Vec3::Z` is used instead

?
Already changed.

[nicopap]

Good job! I think this change is positive.

[alice-i-cecile]

Appears to be no longer breaking :)

[alice-i-cecile]

@lewiszlw can you update the PR description to reflect the final state of this PR?

[lewiszlw]

The default behavior changed when direction can't be normalized. So marked it breaking. @alice-i-cecile