transparentify: Fixed sorting order for alpha
transparentify: Fixed sorting order for alpha

Pairs passed to `compareFn` of `Array.prototype.sort` is implementation specific. While
`node+v8` passes pairs in same order as array i.e. `a = i, b = i + 1`, Edge passes
it in reverse order i.e. `a = i + 1, b = i`. Because of this `node+chakracore` breaks in
`transparentify()` function when `bestAlpha` is sorted. The expectation is sort `bestAlpha`
in descending order and pick the hightest element (at index 0), `node+chakracore` sorts it in ascending order
and gets the lowest element (at index 0).

Updated the `compareFn` to be more agnostic to the implementation of javascript engines.
Ran unit test on `node+v8` and `node+chakracore` and there is no new regression introduced.

stylus/stylus,jangarcia/stylus