Issue Summary

When I was testing the movie import I noticed that playing back an animation with a sound layer from the last frame, repeatedly, that is wait for the animation to end and press the play button again, seems to mess up the caching and shifts the audio back so the video and audio are not synchronized anymore.

Once it ends If I press the "Jump to the Start" button, and play the video/audio sync still gets messed up.

If I play the video but stop it either halfway or about 75% the animation length before it ends by itself, press the "Jump to the Start" button, and then play again the video does NOT get unsynced. If I stop if after that even if it hasn't ended it does get unsynced once i play it back.

Strangely enough, if I let i play with the shifted synchronized and stop it before it reaches the end, but then move the scrub back to the first frame and play again, the playback works properly. If I let it end normally, this doesn't work.

Even more strange, If I use the "Jump to the End" button from the first frame, and then playback from there, the playback does not lose synchronization once it goes back to the first frame.

If I play normally, stop and then use the Jump to the End button, and then play again to it goes back to the first frame, it works as well.

Expected Results

Playback audio should never become unsynced regardless of the frame position the user is located at when they start playback.

Steps to reproduce

Download Sample: https://we.tl/t-DqZDNAr5b7 Link will last a week from the day this issue is submitted so please download it early. If testers can't download, let me know to reupload.

1. Open Pencil2D
2. Open the sample immediately
3. Playback normally Note: Here you'll notice an ungodly lag, the images are getting cached it seems. I'm not sure if this is as normal as it used to be. Play it back various times, no need to let it end, it's just so the lag goes away to force caching. That way you can properly see the video/audio synchronized to normal expectations.
4. Now for the real test: Playback the video once and let it finish. Take note of how the character says "he won't be a boy forever" it is synced
5. The playhead should be on the last frame, now press the Play button again. You will notice the video/audio are unsynced now, it will be noticeable around the same part as point 4.
6. Now before it ends, stop the playback. Press Jump to Start button.
7. Play back again, now you'll see the lag went away and the video/audio are synced again.

System Information

Version: 0.6.5

commit: 50a0c7c
date: 2020-08-01_01:32:15

Operating System: Windows 10 Pro (10.0) V1909 OS Build 18363.959
CPU Architecture: x86_64
Qt Version: 5.12.9