Josep Torra authored Nov 06, 2019 and Haihao Xiang committed Jan 19, 2020

Before this change decoder used the oldest frame in the list to pair it
with the decoded surface. This only works when there's a perfect stream
like HEADERS,SYNCPOINT,DELTA...

When playing RTSP streams we can get imperfect streams like HEADERS,
DELTA,SYNCPOINT,DELTA... In this case decoder drops the frames
between HEADERS and SYNCPOINT which leads into using wrong PTS on
the output frames.

With this change we inject the input PTS into the bitstream and use it
to align the internal frame list with the actually decoded position.

Fixes playback with:
```
gst-launch-1.0 rtspsrc location=... latency=0 drop-on-latency=1 ! ...
```