[kdenlive] defect audio rendering after project migration

[Karl Kiniger]

Hello again,

using the following simple procedure the migration problem can be seen
easily:

1. generate or take some audio, I took 20 seconds of C-major chord from ffmpeg
   (a newer version needed, I used current git), vibrato for better
   listening experience...:

ffmpeg  -f lavfi -i "sine=frequency=262:sample_rate=48000:duration=20" \
        -f lavfi -i "sine=frequency=392:sample_rate=48000:duration=20" \
        -f lavfi -i "sine=frequency=330:sample_rate=48000:duration=20" \
        -f lavfi -i "sine=frequency=524:sample_rate=48000:duration=20" \
        -filter_complex "[0][1][2][3]amerge=inputs=4,pan=1c|c0=c0+c1+c2+c3,vibrato=d=0.2,volume=6dB[aout]" \
        -map "[aout]" -ac 1 \
        -c:a pcm_s16le -y  audio.wav

(you can also use any other audio source but now I understand ffmpeg
filter_graph a little more :-)


2. create a new kdenlive-0.9.10 project and place audio.wav at the
   beginning of an audio track

3. add volume(keyframeable) effect to the audio track, then adjust
   start volume to 0 and volume at end of clip to 100

4. using X (cut) tool, cut the clip into three thirds.
   (I took 3 parts because I wanted to see the ending volume of the
   middle clip)

4a move the first and third part to another audio track keeping the time position
   (perhaps not even needed but I am in a hurry now:-)

5. save the project and import it into kdenlive-16.04, then render it

Result: the linear increasing gain is not preserved properly. The
        first part starts from volume zero but the following parts
        dont continue the continous volume increase.

Kdenlive-0.9.10 renders it without an audible glitch, the volume is
        constantly increasing as intended.

Greetings,
Karl