function transformMat3(out, a, m) {
    var x = a[0],
        y = a[1];
    out[0] = m[0] * x + m[3] * y + m[6];
    out[1] = m[1] * x + m[4] * y + m[7];
    return out;
  }
Transforms the vec2 with a mat3
3rd vector component is implicitly '1'

@param {vec2} out the receiving vector
@param {ReadonlyVec2} a the vector to transform
@param {ReadonlyMat3} m matrix to transform with
@returns {vec2} out