[MSE] Timestamp offset mechanism

Hi,

Bug 17004 <https://www.w3.org/Bugs/Public/show_bug.cgi?id=17004> proposes a
mechanism for adjusting timestamps inside media segments. This enables
seamlessly splicing content together that doesn't originally exist on a
common presentation timeline. Inserting advertisements into content is the
most straight forward use case. There has been some useful discussion in
the bug comments, but we haven't decided on which method signature to use
for specifying the timestamp adjustments.

Here are the proposed signatures at this point:

void sourceTimestampMapping(in double presentationTimestamp, in double
segmentTimestamp)
 -- or --
void sourceTimestampOffset(in double timestampOffset)

sourceTimestampMapping() just provides a way to say, "I want this timestamp
in the segment to map to this presentation time."
sourceTimestampOffset() is basically just saying, "Add this offset to any
timestamp in the segment to get the desired presentation time."

Fundamentally they do the exact same thing, but I'm looking for input on
which option people feel more comfortable with.

Which do you prefer?
Are you ok with the semantics outlined in the bug
description<https://www.w3.org/Bugs/Public/show_bug.cgi?id=17004#c0>
?

Aaron