libgdiplus new formula

> Are you sure none of those are installed by Mono itself already and just buried somewhere?

As far as from what I see, all of them have their own git repository. While homebrew mono is built from mono-core sources. So they are not buried somewhere, despite they should be in the mono runtime.

I'm more concern that there are other libraries missing. So I installed both homebrew mono and `mono-mre` from cask and compare the `.dylib` files installed by them. [This](https://gist.github.com/xu-cheng/9fb8d617ec52cc746ace) is the result. Most of diff files come from 3rd party dependencies, while some are the file missed by homebrew mono.