Update screenshot logic to handle blurred views.
Update screenshot logic to handle blurred views.

By using the `drawHierarchy(in:afterScreenUpdates:)` method of
`UIWindow`, we can draw the entire view hierarchy of the app’s window,
including the blurring caused by `UIVisualEffectView`s.

Fixes #28.
detroit-labs/BugShaker,detroit-labs/BugShaker,detroit-labs/BugShaker