export mp4 degradate images

Siril version: 0.99.6

Note: commit aaaa12a (from siril main screen)

Operating System: Windows 64bit

Package: website

Description of the bug

when a sequence is exported to mp4 (without normalization, see #602 (closed)) images are "degradated"

an example of an image from the sequence before exportation:

after export the same image: (as you can see it is always the same image of the sequence, number 55)

the exported mp4: https://streamable.com/4axauz