mpegtspacketizer: Avoid usage of GstBuffer internally

mpegtspacketizer: Avoid usage of GstBuffer internally

The overhead of creating/using 188 byte GstBuffer from GstAdapter
is too expensive.
We now peek the next packet, and provide a data/size which is only
valid until the packetizerpacket is cleared.

In addition, cleanup all the internal code to deal with that new
behaviour and remove double-checks which are no longer needed.

The section_length is now the corrected section_length (i.e. with
the additional 3 bytes).

Avoid using gst_adapter_prev_timestamp and instead track
the timestamp ourself.