Fix: Width/Height from Screen

• Switched to reading the width and height directly from `UnityEngine.Screen` instead of indirectly from `UnityEngine.Camera.main`.
	» This fixes NullReferenceExceptions in the rare-but-plausible case that there is no Camera in the scene.  This is known to occur between levels, and especially when using additive level-loading techniques.
	» Using a Camera's width/height only makes sense if the Camera is settable somewhere.  To hard-code it to Camera.main is no different than using the Screen values unless the main Camera were to be displayed on only part of the screen, which would be really weird, and probably not the touch input area we want anyway.
	» Reading directly from Screen also ensures we always are using the current values, if the Screen's dimensions were to change (such as when rotating).
	• TuioTrackingComponent and WM_TrackingComponent updated to use `Screen.width`/`.height`.
	• `ScreenWidth`/`Height` public vars removed from TuioComponentBase— we no longer need or want to cache this.
	• Removed InitTracking() and contained set of `ScreenWidth`/`Height` from TuioInput, MouseInput, and WM_Input.
		» The `InitTracking()` wasn't doing anything else and was mis-named— it wasn't init-ing, it was only config-ing.  Since I don't know what other intent it may have been put in for (no comments, inaccurate name), I'm removing it.