Add crouch controls

[trusktr]

This is WIP, currently holding Shift makes the camera keep going downward, but instead we want to go to a lower height while holding Shift, then go back to normal height when we let go.

[trusktr]

cc @ryyeung we're almost done. Now that jumping controls are in, we can do something simple with crouch for now: if shift is held, set player height to a smaller number, otherwise set back to default height.

[trusktr]

Ensure that if the window loses focus while Shift is held that the crouch won't get stuck.