Fix for b2PolygonShape::ComputeDistance

[Manamongods]

This makes the distance correctly calculated when it comes to corners.

[ryanpeach]

@Steffenvy how has your recent experience been with the library? I'm choosing to start with it but it appears unmaintained. Would your repo be a good place to focus new efforts on bug fixes and modernization?

[Manamongods]

@ryanpeach The only experience I have with the library is using it with swig c# wrappers (so I could use it in unity) and then adding a lot of features and changes that I need. It has been fun, most of the time. I dont know else to say though.

I'm unsure much difference it makes, because my commits are only bug fixes that I manually plucked from my own heavily modified repo. I'd likely accept pull requests, but I probably won't make sure they work first, I mean I don't even really know how that process normally goes.
I consider these commits to be an important starting point though, and if somebody finds bugs with the particle simulation particularly, I would be very interested, although ultimately liquidfun works for my purposes as it is now, in terms of bug fixes, and additional changes to either repo would be rather difficult to transfer to the heavily modified repo that I use anyways.
It's probably best to keep the bug fix commits in this repo so that people who want to use liquidfun can find them in the pull requests section, whereas my repo is difficult to find or know about, although I am pretty new to git in general so I'm not sure.

As for modernization, I'd like to know what you mean, like features? Or support for something?

[ryanpeach]

modernization as in keeping up with box2d.
I suppose as long as the repo works its fine, It just worries me getting started on something that hasn't seen an update in 9y, whereas it's base box2d had an update 3y ago.
I'm also new to C++ though and coming from python that's almost unheard of, but maybe in C++ things are stable enough to leave alone for long periods of time.

[Manamongods]

Personally I'm unfamiliar with box2D's updates, it might be good to upgrade liquidfun's box2D, but it's probably best to only do that if required or for the fun of it