Fix timestamp overflow issues

Fix timestamp overflow issues

- Save the initial user-specified timestamp and rebase all further
timestamps by this value. This makes libvpx internal timestamps to
always start from zero, regardless of the user's timestamps.
- Calculate reduced timestamp conversion ratio and use it to convert
user's timestamps to libvpx internal timestamps and back. The effect
of this is that integer overflow due to multiplication doesn't
happen for a much longer time.

BUG=webm:701

Change-Id: Ic6f5eacd9a7c21b95707d31ee2da77dc8ac7dccf