rtpgstpay: Don't push packets before the first input buffer is received
It's not possible to create a valid RTP timestamp for them, which would cause a potentially very big RTP timestamp discontinuity between those first packets (created from initial events) and the packet based on the first input buffer.
As a side-effect, also simplify the packet aggregation code a bit and work with only a single level of buffer lists.
Fixes #1157 (closed)