code: public GVRTransform setModelMatrix(float[] mat) {
          if (mat.length != 16) {
              throw new IllegalArgumentException("Size not equal to 16.");
          }
          NativeTransform.setModelMatrix(getNative(), mat);
          return this;
      }
docs: Set the 4x4 model matrix and set current scaling, rotation, and
  transformation based on this model matrix.
  
  @param mat
  An array of 16 {@code float}s representing a 4x4 matrix in
  OpenGL-compatible column-major format.