improve mousedrag movements in FreeFlight Controls

[cvan]

originally identified in PR #213 (for issue #125) in this comment: #213 (comment):

not sure if it's coming from the YawMovement or PitchMovement, but the mousedrag (especially at lower regions of the screen) doesn't seem to rotate the camera correctly; it's like a lag or stutter

[caseyyee]

Yeah, sem to get stuttering and some inconsistencies when dragging.

Maybe something more along the lines of:
https://threejs.org/examples/misc_controls_pointerlock.html

It's a bit different in that it's not freeflight, but the mouse-locked, look and fps style controls seems a bit more what I would expect in terms of navigating a standard scene.

[cvan]

I also notice that the controls keep moving even after I have released a mouse click. I would agree that I would expect FPS Pointer-Lock as a default, preferred camera controls. Free-Flight is useful, but I'm not sure the demo scene is a good example of that. (out of curiosity, what are some examples of scenes where Free-Flight would be preferred?)