Added methods to the Camera class for getting/setting horizontal FOV

Field of view is usually specified for the vertical angle, because it is independent of the aspect ratio. But calculating the vertical field of view for a given horizontal field of view is non-trivial. That's why I've added methods for getting/setting the horizontal field of view of the camera.

I thought about adding a parameter to the existing getFov() and setFov() methods, but decided a separate method would be better.