Bug 511138

SUMMARY
Everytime after I add subtitles in anyway - either myself or using the speech recognition feature I get an error in the UI saying `[filter avfilter.subtitles ] Unexpected return format` when I click on the play button or manipulate the timeline in any way. And as a result the subtitles don't get rendered, rendering them useless.

When I run kdenlive from the terminal and do the same action I get this message:
```
[swscaler @ 0x7f35bc028680] Unsupported input (Operation not supported): fmt:yuyv422 csp:bt709 prim:reserved trc:reserved -> fmt:yuv422p csp:unknown prim:reserved trc:reserved
MLT: "[filter avfilter.subtitles ] Cannot get frame from buffer sink"
MLT: "[filter avfilter.subtitles ] Unexpected return format"
```

Also, when I try to render the project with the subtitles, the rendering process crashes with the error log being just the following message on repeat: 

```
[swscaler @ 0x7fdc341d3340] Unsupported input (Operation not supported): fmt:yuyv422 csp:bt709 prim:reserved trc:reserved -> fmt:yuv422p csp:unknown prim:reserved trc:reserved [filter avfilter.subtitles] Cannot get frame from buffer sink [filter avfilter.subtitles] Unexpected return format
``` 

STEPS TO REPRODUCE
1. Create a new project
2. Add something to the timeline and ad  
3. 

OBSERVED RESULT


EXPECTED RESULT


SOFTWARE/OS VERSIONS
Linux/KDE Plasma: Arch/Not using KDE Plasma
KDE Plasma Version: Not using KDE Plasma
KDE Frameworks Version: 
Qt Version: 5.15.17