libgdiplus new formula

> 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?
> How to test a gui app? Even if I succeed to build a working gui program using this library, how do I test it in the CLI environment?
> 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.

All we need to do for this test is import a public symbol and link to it. If this should be part of the Mono runtime then it probably shouldn't be a separate formula.