Amethyst: Fly Camera rotates automatically when window dimensions are odd

[Lukazoid]

When the window dimensions are odd the fly camera will rotate without any mouse input.

This can be demonstrated with the fly_camera example by adding the following to display_config.ron:

dimensions: Some((801, 600)),

You should see the camera rotate automatically around the vertical axis.

[jojolepro]

You should think about becoming our dedicated quality assurance :)

[Lukazoid]

Haha, I'm just playing around with the fly_camera and noticed this while trying to investigate #636.

I think this is caused because the MouseCenterLockSystem is locking to the integer center while FreeRotationSystem uses the floating center to determine the delta, thus for a screen with odd dimensions there is always a delta of 0.5.

I am happy to attempt to fix this later once I've finished work for the day.