Final fixes for 0.99.8

After more testing, several issues arose and need to be fixed before the release:

1. bitrate can be 0 apparently in mp4 export, already in ticket #603 (closed)
2. some FITS header keywords that are normally handled by siril are lost in the processing
3. 16-bit FITS images stacking changed since last version and does not produce good results (rejection is very different compared to other file types)
4. two converts to SER in a script seem to crash siril
5. SER stacking with 32 bits output seems to result in an incorrect file format
6. update release notes for all the above if needed (for 5, it's a long time bug so it should be added: fixing preprocessing of SER sequences with 32-bit master files).