Support UIAppearance and visual effect views

[stephencelis]

iOSSnapshotTestCase supports UIAppearance and visual effect views using a window and drawViewHierarchyInRect:

https://github.com/uber/ios-snapshot-test-case/blob/de1c714952ca6b7f1a40670135297fd761ba43cf/FBSnapshotTestCase/Categories/UIImage%2BSnapshot.m#L39-L70

We typically write tests against framework, not app, targets, where I'm not sure this is even possible. We should investigate support, though, maybe at least by introspecting whether or not there's a window we can use.