Position shift when looking straight up or down

[fenhl]

When looking straight up or down, the position of the camera appears to shift forward by about 2 pixels.

[RichardlL]

Pull request:

https://github.com/PistonDevelopers/hematite/pull/250

[bvssvni]

Fixed by https://github.com/PistonDevelopers/hematite/pull/250

[eddyb]

Wrong fix, the problem is that camera.forward can have x and z equal to 0, i.e. looking straight up or down, which is... maybe not incorrect, but the camera still needs to always preserve a horizontal direction to be minecraft-like. Which would be the yaw, I believe. Extracting that and using it for the xz_forward offset #250 effectively removed should do the trick.