videodecoder: handle buffer's flags at offset

videodecoder: handle buffer's flags at offset

For reverse playback it is important to handle correctly the frame sync
points, which is set when the input buffer doesn't have the DELTA_UNIT flag.

This is handled correctly when decoder is packetized, but when it is not the
frame's sync point is not copied, and the reverse playback never decodes frame
batches.

The current patch adds the buffer's flags to the Timestamp list, where the
timestamp and duration of the input buffers are hold.