Added Reset method to TwopassStatsStore

The stats buffer needs to be reset between runs of the
encoder. I added a Reset() function to TwopassStatsStore
and called it at the beginning of each encode.

This enables us to run multiple encodes which was
previously not possible since there was no way to reset
the stats between runs.

Change-Id: Iebb18dab83ba9331f009f764cc858609738a27f9