/*
** 行列 m0 と m1 の積を求める
*/
void multiplyMatrix(const GLfloat* m0, const GLfloat* m1, GLfloat* matrix)
{
  for (int i = 0; i < 16; ++i) {
    int j = i & ~3, k = i & 3;

    matrix[i] = m0[j + 0] * m1[0 + k]
      + m0[j + 1] * m1[4 + k]
      + m0[j + 2] * m1[8 + k]
      + m0[j + 3] * m1[12 + k];
  }
}