When a WinStore app becomes hidden, forget any held keys.

If they are released while we aren't active we won't know about it, so we get out of sync.

This can be tested with the platformer sample. Hold ```d``` to start moving right. Press ```alt-tab``` to leave the app, release ```d```, press ```alt-tab``` to go back to the app. You'll still be moving to the right

Fixes #3212