adapter: Rename functions and implement new functions, update test

adapter: Rename functions and implement new functions, update test

We don't do calculations with different units (buffer offsets and bytes)
anymore but have functions for:
1) getting the number of bytes since the last discont
2) getting the offset (and pts/dts) at the last discont

and the previously added function to get the last offset and its distance from
the current adapter position.

https://bugzilla.gnome.org/show_bug.cgi?id=766647