Bug fixes about topViewSnapshot's frame & shouldAllowPanningPastAnchor

Hi, @edgecase. In these commits I try to fix some problems I encountered when using ECSlidingViewController within my project:

1. The top view snapshot's frame doesn't get updated. This makes the snapshot fail to cover the entire top view after the topViewController anchored either side sometimes. (e.g. If the app supports landscape mode only)
2. The top view stuck before anchor point problem when shouldAllowPanningPastAnchor is set to NO.