Support a mask2k file to prioritize sections VERSION 2

[Alch-Emi]

This is a minor rewrite of @octylFractal's #3.

The most significant change is to the algorithm which decides which pixel to draw from a list of buckets. Where octyl's method groups pixels into buckets, selects a bucket using a weighted random, and then selects a pixel from that bucket at a uniform random, the revised algorithm creates a virtual pool of 150 pixels pulled from the highest priority buckets, and then selects evenly from that pool.

This strategy further reduces the chance of a collision and also provides a single number that can be used to balance strength of prioritization against collision resistance.

This also patches most of the nits that were brought up in the original pull request.

I've done my best to review the code on my own, and am running + observing it in testing, but code review is still strongly requested, if possible.

Thanks for any feedback <3

[bb010g]

Please rebase this on top of d/main.