Expose `[GVRUIViewRenderer hitTest:(GVRHeadPose*)]` as a public API

[allenhumphreys]

1. This function is dead useful, and could probably be considered as a candidate for GVRRenderObject
2. The math it performs is non-trivial and that alone makes it super helpful
3. Keeping it private just means I have to do extra work to call it

[Performador]

Good idea! It will be public in the next release.