Fix FPS camera movement bug

[dr-nyt]

In chapter 1, the first exercise from section 10.10 asks to convert the camera to be a true fps camera, with no vertical movement. The original code simply sets the y axis of the position to 0 to block vertical movement, but there is a bug where if the player is looking directly up or down, the player will slow down to a halt because it is the vertical vector from the pitch is added to the position.

I fixed this by creating a new vector that only adds the yaw (horizontal) axis while leaving the y-axis to 0. Less computation as well.