bug 1044125 - Only capture screenshot and show overlay when edge gesture...

...s are in use or task manager is used. r=alive
I think there's a better way.
Multiple edge swipes can happen in a row, and we'll end up decoding/trashing the screenshots for each swipe. Making the perf even worse.

We already have a mechanism in stack_manager.js to "batch" those subsequent swipes and only dispatch events for one app change.
If your stack is `[A]-[B]-[C]-[D]`, and you go from `D` to `A` by swiping fast we'll only broadcast that `[D]` is now hidden and `[A]` is now displayed. Not the intermediaries.

In the same fashion I think we should drop the screenshots only when we broadcast the app change in stackmanager and not at the end of every swipe.