Fix presentationTimeOffset handling in v2

[joeyparrish]

We aren't handling presentationTimeOffset correctly yet. Here's an snippet of a manifest we aren't handling right:

    <Period id="0" start="PT0.000S">
        <AdaptationSet mimeType="audio/mp4" segmentAlignment="true">
            <SegmentTemplate timescale="22050" presentationTimeOffset="2106635264"
                    initialization="dash-init-0.m4a"
                    media="dash-$Time$.m4a">
                <SegmentTimeline>
                    <S t="2106635264" d="109568" />
                    <S t="2106744832" d="110592" />
                    <S t="2106855424" d="61440" />
                </SegmentTimeline>
            </SegmentTemplate>

The PTO is the same as the first segment's timestamp. In the v2-work-in-progress today, this results in the error "STREAMING.SEGMENT_DOES_NOT_EXIST (audio,0,1)".

[joeyparrish]

@perses, JFYI

[ct1n]

Thanks!