Encapsule image processing inside mcSkin struct

Adjusted some stuff. Image processing is done within a struct, mcSkin, which inherits from minecraft.Skin. This is slightly "nicer" design, I believe, and more importantly allows us to easily store information about the image we're processing.

I originally came here to make an isometric head view for this day of [24pullrequests](https://24pullrequests.com). This paves it way for that: it allows us to, for example, gracefully handle more fine-tuned resizing of avatars (soon to come!) beyond the currently always-on nearestNeightbor, without adding additional complexity inside the http mux.