Working on joystick support using SDL.
Avoid unused variable warnings in Xcode.
First cut at adding external joysticks to the mix.
Merge branch 'master' of https://github.com/highfidelity/hifi into priority
Fixed index of out bounds error, issue with head not being tracked in
PrioVR.
Basic joystick integration.
Merge branch 'master' of https://github.com/highfidelity/hifi into priority
Lock joystick states for thread safety.
Scratch that; going to shoehorn joystick data into palms.
Stuff the joystick and position bits into the palms.
Merge branch 'priority' of https://github.com/ey6es/hifi into priority

Conflicts:
	interface/src/Application.cpp
	interface/src/devices/PrioVR.h
	interface/src/scripting/ControllerScriptingInterface.cpp
More joystick fixes.
Merge pull request #2870 from ey6es/priority

Basic support for joysticks using SDL, shoehorn PrioVR position/joysticks into palm data.