Mono: fix url and add libgdiplus

Per conversion: https://github.com/Homebrew/homebrew/pull/34955. This PR fixes the `monolite` url and adds the missing `libgdiplus`.

As for `gtk-sharp`, `gtk-quartz-engine` and `mono-tools`, I haven't got them working yet. Some due to upstream bug, other for missing dependencies. 

There should also be other missing libraries for mono runtime. But I'm not familiar to mono. And there're so many libraries under https://github.com/mono. I don't know which of them should be put into mono runtime.

In fact, I would recommend users install mono by cask rather than homebrew for now.
Probably worth adding a comment to specify that these are only needed for `libgdiplus`