Add edge handling to getPixelIndex
Add edge handling to getPixelIndex

Allows to find related pixel passed on edge handling strategy.

* Extend: uses the nearest pixel.
* Wrap: uses the opposite side pixel, like a tilled background.
* Crop: the current behavior.

oliver-moran/jimp,oliver-moran/jimp,skoa-h/jimp,oliver-moran/jimp,skoa-h/jimp,skoa-h/jimp