Bug 511139

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"
```

Attempting to render the project with subtitles also causes the rendering process to crash, repeating the same error messages. The render job still produces an output file, but the subtitles are missing from the result.

```
[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
```

Even though the rendering job crashes, producing a rendered file. After inspecting the file, the subtitles are not present in it as well.

STEPS TO REPRODUCE
1. Create a new project
2. Add a video clip to the timeline
3. Add a subtitles track
4. Add subtitles (manually or via speech recognition)
5. Press Play or Render the project

OBSERVED RESULT
- The subtitles aren't rendered in the preview, displaying the error message `[filter avfilter.subtitles ] Unexpected return format`. 
- When rendering the project with any subtitles in the timeline, the rendering job crashes with error log containing a similar message on repeat.
- Even though the rendering crashes, it still produces an output file in which the subtitles are not present

EXPECTED RESULT
- The subtitles are rendered to the preview when added to the timeline
- The user is able to manipulate the subtitles in any way, always rendering them correctly
- The rendering job doesn't crash when there are subtitles present in the timeline
- The subtitles are present in the final rendered output file

SOFTWARE/OS VERSIONS
Linux/KDE Plasma: Arch Linux (non-KDE Plasma environment)
KDE Plasma Version: -
KDE Frameworks Version: -
Qt Version: 5.15.17
Kdenlive Version: 25.08.2