libgdiplus new formula

I can go to have a try. But, there're several questions ahead of time.

1. The test case would depend on mono, while the package itself doesn't. So I need put `depends_on "mono"` inside the test block. Am I right?
2. How to test gui app? Even if I succeed to build a working gui program using the library, how do I test it in the CLI environment.
3. The library should be part of mono runtime. But for some unknown reason, homebrew mono doesn't include this. And it seems that there're another missing libraries in homebrew mono.