fix timestamp calculation rollover w/altref

using large values for the timebase, e.g., {33333, 1000000} could
rollover the timestamp calculation in vp8e_encode as it was not using
64-bit math.

originally reported on ffmpeg's trac:
  https://ffmpeg.org/trac/ffmpeg/ticket/1014

BUG=468

Change-Id: Iedb4e11de086a3dda75097bfaf08f2488e2088d8