Add `FirstPerson::look_at`

[bvssvni]

Currently, there is no built-in method for FirstPerson to look at a particular point.

Here is some code I've been using (might require testing):

/// Calculates the yaw and pitch to look at something for first person camera.
pub fn yaw_pitch(
    pos: Vec3,
    look_at: Vec3
) -> (f64, f64) {
    use vecmath::*;
    use vecmath::traits::Radians;

    let d = vec3_normalized(vec3_sub(look_at, pos));
    let yaw_z = d[2].atan2(-d[0]);
    let yaw = yaw_z - f64::_90();
    let p = [-yaw_z.cos(), 0.0, yaw_z.sin()];
    let pitch = vec3_dot(d, p);
    (yaw, pitch.acos())
}