libvpx

Extend --auto-alt-ref so it can enable multi-alt ref.

Extend --auto-alt-ref from parameter so we can use it to
turn multi-arf on and off from the command line.

For now the range is 0-off, 1-on, 2-multi-arf on.

Rename play_alternate to enable_auto_arf

Change-Id: Id7b64407cfbe76ba0090a83b588a03e22a240386