[rtspsrc/jitterbuffer] position increasing while buffering

Submitted by Mark Nauwelaerts @mnauw

Link to original bug (#644404)

Description

Jitterbuffer's buffer mode performs buffering by simulating a PAUSE using timestamp offset (in concert with rtpbin). The ts offset is needed as it is only a simulation since PLAYING state is maintained, and as such running time continues to increase. Consequently, position reporting (derived from running time) also increases while buffering is ongoing (and also off the mark when resuming). This is particularly confusing for buffer mode's primary use case, i.e. non-live material (where position has a very specific meaning).

Unfortunately, jitterbuffer's slave mode (still?) does not handle bursts well, unless there is some room for tweaking there ...