Add ALT_MIN_LAG
Add ALT_MIN_LAG

This is the minimum lag-in-frames to allow auto_alt_ref.

BuG=aomedia:648

Change-Id: Idb5eb839463e8257de1cce9c8d6590409d0cc60c

GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom