Fix: Prevent backdrop press when a modal is hiding

[rossmartin]

There is an issue where you're able to get a backdrop press in for a modal while it is hiding. When doing this it appears that the internal state of the rendered modal stack gets messed up.

Here is a scenario where this can be reproduced:

1. Call openModal for modal "A" and then call closeModal for it while repeatedly pressing the backdrop
2. In the closeModal callback of modal "A" call openModal for modal "B"
3. Observe that modal "B" never shows but currentModal is truthy and modal "B" should be showing
4. Let's say you call openModal for modal "C" at this point - you'll very briefly see modal "B" before modal "C" shows

This can be easier to reproduce by setting the backdropAnimationDuration to a higher value. That makes it easier to get a press on the backdrop while it is hiding.

Thank you for this great library! Please let me know if this is enough to illustrate the issue.