Fix alpha bleed

[EgoMoose]

This PR makes a couple of fixes / adjustments to the alpha bleed algorithm.

1. It only bleeds one pixel now. See this README for why.
2. Fully transparent pixels that are not bled into are converted to pure black for better encoding.
3. Fixes a mistake in the algorithm where pixels could sample neighbors from the same queue cycle.

The first pass iterates over pixels left-to-right, top-to-bottom so this effect naturally impacts pixels that come further along in the iteration (right and bottom). However, due to the order of the neighbor offsets this effect is somewhat muddied. Regardless, pixels should not be sampling their bled neighbors from the same cycle.

Given an input image:

The results go from looking like this (everything fully opaque):

To this: