code: function _mmult(a, m) {
          m = m.slice();
  
          var m0 = m[0];
          var m1 = m[1];
          var m2 = m[2];
          var m3 = m[3];
          var m4 = m[4];
          var m5 = m[5];
          var m6 = m[6];
          var m7 = m[7];
          var m8 = m[8];
  
          m[0] = a[0] * m0 + a[1] * m3;
          m[1] = a[0] * m1 + a[1] * m4;
          m[3] = a[3] * m0 + a[4] * m3;
          m[4] = a[3] * m1 + a[4] * m4;
          m[6] = a[6] * m0 + a[7] * m3 + m6;
          m[7] = a[6] * m1 + a[7] * m4 + m7;
  
          return transform(m);
      }
docs: Performs the 3x3 matrix multiplication of the current matrix with the input matrix a.