fix(AdaptiveQuality): ensure adaptive quality works on CameraRig
fix(AdaptiveQuality): ensure adaptive quality works on CameraRig

#661 changed the way scripts are added to the `CameraRig` or any of its
children. The problem is that `AdaptiveQuality` uses the `OnPreCull`
method which is only called if the script is added on a game object
that has a `Camera` attached to it. This change fixes the problem by
adding an invocation to the `Camera.onPreCull` delegate and ignoring
all other cameras except the headset camera.

gpvigano/SteamVR_Unity_Toolkit,TMaloteaux/SteamVR_Unity_Toolkit,virror/VRTK,thestonefox/VRTK,adamjweaver/VRTK,red-horizon/VRTK,Innoactive/IA-unity-VR-toolkit-VRTK,Fulby/VRTK,gomez-addams/SteamVR_Unity_Toolkit,nmccarty/htvr,mattboy64/SteamVR_Unity_Toolkit,gpvigano/VRTK,pargee/SteamVR_Unity_Toolkit,thestonefox/SteamVR_Unity_Toolkit