Fix alpha blending
Adding this issue to brainstorm a fix. Related to BUG: 422918. Apply fix to qtblend maybe?
According to Aurélien Pierre from Darktable: "You can't do alpha blending properly in 8 bits uint because gamma needs to be decoded for alpha to mean occlusion as in the Porter & Duff paper. The consequence is most of the transitions are pretty ugly and ill-blended."
More info about alpha compositing: https://en.wikipedia.org/wiki/Alpha_compositing#Gamma_correction
Possible solution https://t.me/kdenlive_dev/12877:
- step one would be to use at least float16
- then undo gamma for every RGB
- then compose, then redo gamma on RGB
- also it composes in YUV, which is its own problem
Issue reported upstream: https://github.com/mltframework/mlt/issues/864