Merge "Clarify optionality of head tracker velocity"

             * velocity of the head (relative to itself), in radians per second.

             * The direction of this vector indicates the axis of rotation, and

             * the magnitude indicates the rate of rotation.

             * If this head tracker sensor instance does not support detecting

             * velocity, then these fields must be set to 0.

             */

            float vx;

            float vy;