toQuat
Converts this EulerAngles instance into its equivalent Quat (quaternion) representation.
The conversion is based on the specific extrinsic ZXY rotation order that this class represents. This means the rotations are applied sequentially around fixed world axes:
First, a Roll around the world Z-axis.
Second, a Pitch around the world X-axis.
Finally, a Yaw around the world Y-axis.
The resulting quaternion represents the same final orientation as these Euler angles.
Return
A unit Quat that represents the rotation.