Detect looking at objects

Wait, there's more! Just one more thing to add. Building interactive applications require us to be able to determine whether the user is gazing at a specific object. We can put this into RenderObject, so any objects in the scene can be gaze detected.

The technique that we'll implement is straightforward. Considering each object we render is projected onto a camera plane, we really only need to determine whether the user is looking at the object's plane. Basically, we check whether the vector between the camera and the plane position is the same as the camera's view direction. But we'll throw in some tolerance, so you don't have to look exactly at the center of the plane (that'd be impractical). We will check a narrow range. A good way to do this is to calculate the angle between the vectors. We calculate the pitch and yaw angles between these vectors (the up/down X axis angle and left/right Y axis angle, respectively). Then, we check whether these angles are within...