qctools player works in context of thumbnails from qctools report rather than source file

[dericed]

sample file: ffmpeg -f lavfi -i color=r=16:s=1024x1024 -vf format=yuv444p10le,geq=lum=X:cb=512:cr=512 -t 1 -c:v ffv1 1024gradient.mkv

When opened in qctools 1.3, it looks like:

Then save the report and reopen:

In the before images, the image format is yuv422p10le (10 bit).
In the after images, the image format is yuvj420p (8 bit). This refers to the thumbnails.
But in the after images, the waveform is different, flat at the left/right ends instead of an intended diagonal.

[dericed]

See also related discussion and reporting by @alec-bertoy at #810