code: public Matrix4f rotationX(float ang) {
          float sin, cos;
          sin = (float) Math.sin(ang);
          cos = (float) Math.cosFromSin(sin, ang);
          if ((properties & PROPERTY_IDENTITY) == 0)
              MemUtil.INSTANCE.identity(this);
          this._m11(cos);
          this._m12(sin);
          this._m21(-sin);
          this._m22(cos);
          _properties(PROPERTY_AFFINE | PROPERTY_ORTHONORMAL);
          return this;
      }
docs: Set this matrix to a rotation transformation about the X axis.
  <p>
  When used with a right-handed coordinate system, the produced rotation will rotate a vector
  counter-clockwise around the rotation axis, when viewing along the negative axis direction towards the origin.
  When used with a left-handed coordinate system, the rotation is clockwise.
  <p>
  Reference: <a href="http://en.wikipedia.org/wiki/Rotation_matrix#Basic_rotations">http://en.wikipedia.org</a>
  
  @param ang
  the angle in radians
  @return this