look_at is broken

[sognefej]

failures:

---- mat4::tests::mat4_look_at stdout ----
thread 'mat4::tests::mat4_look_at' panicked at 'assertion failed: `(left == right)`
  left: `[0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0]`,
 right: `[0.0, NaN, 0.0, 0.0, 0.0, NaN, 1.0, 0.0, 0.0, NaN, 0.0, 0.0, -0.0, NaN, -0.0, 1.0]`', src/mat4.rs:2731:9
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace.


failures:
    mat4::tests::mat4_look_at

test result: FAILED. 318 passed; 1 failed; 9 ignored; 0 measured; 0 filtered out