function fromMat4$1(out, a) {
    out[0] = a[0];
    out[1] = a[1];
    out[2] = a[2];
    out[3] = a[4];
    out[4] = a[5];
    out[5] = a[6];
    out[6] = a[8];
    out[7] = a[9];
    out[8] = a[10];
    return out;
  }
Copies the upper-left 3x3 values into the given mat3.

@param {mat3} out the receiving 3x3 matrix
@param {ReadonlyMat4} a   the source 4x4 matrix
@returns {mat3} out