Datavyu 1.5.1-RC2 backwards playback is very jittery

[jperricone]

Describe the bug
when setting the playback to negative speeds, the video becomes very jittery

To Reproduce
Steps to reproduce the behavior:

1. Add a video
2. hit play
3. set the playback speed to -1/4x e.g.
4. the video becomes jittery and jumps rapidly between frames

Expected behavior
the video is expected to smoothly playback at specified speed

Desktop (please complete the following information):

• OS: mojave 10.14.4