code: public Quaternion fromAnglesXZ (float x, float z) {
          float hx = x * 0.5f, hz = z * 0.5f;
          float sx = FloatMath.sin(hx), cx = FloatMath.cos(hx);
          float sz = FloatMath.sin(hz), cz = FloatMath.cos(hz);
          return set(cz*sx, sz*sx, sz*cx, cz*cx);
      }
docs: Sets this quaternion to one that first rotates about x by the specified number of radians,
  then rotates about z by the specified number of radians.