Fix breakout thresh computation for golden & AltRef frames

1. Unavailability of each reference frame type should be tested
independently,
2. Also, only the VP8_GOLD_FLAG needs to be tested before setting
golden frame specific thresholds, and only VP8_ALT_FLAG needs
testing before setting thresholds relevant to the AltRef frame.
(Raised by gbvalor, in response to Issue 47)

Change-Id: I6a06fc2a6592841d85422bc1661e33349bb6c3b8