Fix memory allocation

[haydenbleasel]

jimp-dev/jimp#153 (comment)

Everything Jimp or underlying modules (like png-js or jpegjs) do takes time to process and isn't asynchronous (except reading/writing from fs). So it doesn't really matter if you start generating icons in parallel or in sequence - either way we are waiting untill we're done iterating over pixels of the one image untill we can start iterating pixels of the other. On the other hand when V8 sees high memory usage it's trying to clear garbage, but there's nothing to clear, so I would speculate that we're actually wasting some processing time here while GC runs.

On a dirt cheap laptop sequential generation was only 2-3 seconds slower (~14 seconds instead of 16-17) and I would say it's probably caused by reading from fs. But memory usage was 70% lower. I'd say worth it. Keeping copy of the image in-memory and cloning it would definitely make things faster because decoding PNGs is expensive task and it will be run 40 times less often