We have to reverse all samples in a buffer before processing them to properly
have continuous data from one buffer to another. As a result we will have a
negative applied rate and a rate of 1.0.

Also make sure that input buffers are correctly clipped to the segment,
otherwise our calculations are going to go wrong.

Also copy over the segment event's sequence number to the output segment while
we're at it.

https://bugzilla.gnome.org/show_bug.cgi?id=757033