function findEnclosingVertexAngles(a, vangles) {
    var minFn = function(v) {
        var adelta = angleDelta(v, a);
        return adelta > 0 ? adelta : Infinity;
    };
    var i0 = findIndexOfMin(vangles, minFn);
    var i1 = Lib.mod(i0 + 1, vangles.length);
    return [vangles[i0], vangles[i1]];
}
find vertex angles (in 'vangles') the enclose angle 'a'

@param {number} a : angle in *radians*
@param {array} vangles : angles of polygon vertices in *radians*
@return {2-item array}