Previously the HUD fading in/out would also recenter the hmd sensor and the avatar, which caused many problems including:
* The user's view could shift vertically.
* Your avatar would briefly go into t-pose
* other users would see your avatar go into t-pose.
Now we now move the UI sphere instead, which results in a much smoother experience.
MyAvatar: added hasDriveInput method.
OverlayConductor:
* removed avatar and sensor reset, instead the overlay's modelTransform is changed.
* revived STANDING mode, which is active if myAvatar->getClearOverlayWhenDriving() is true and you are wearing an HMD.
* SITTING & FLAT mode should be unchanged.
* Instead of using avatar velocity to fade out/fade in the hud, We use the presense or absanse of avatar drive input.
* Additionally, we check distance to the UI sphere, and quickly recenter the hud if the users head is too close to the actual hud sphere.
CompositorHelper:
* Bug fixes for ray picks not using the modelTransform.
HmdDisplayPlugin:
* Bug fixes for rendering not using the modelTransform.
Before this fix, a script could call into HMD.getHUDLookAtPosition2D() while the app was shutting down, which in turn would call
getHeadPose() on the currently active display plugin. This call could cause a crash within the openvr plugin, because the SDK was either shutdown, or in the process of shutting down on the main thread.
This fixes this by spliting the previous DisplayPlugin::getHeadPose(int) into two parts:
* updateHeadPose(int) which is only called once a frame and only by the main thread.
* getHeadPose() which is thread-safe and will return a cached copy of the hmd pose sampled by the last updateHeadPose.
