Blender Git Loki

Fixes for sequencer effects

- Add and subtract effects will now affect on only RGB channels,
and alpha of first input is used as an alpha for the result.

Also solved creepyness with straight->premul->straight conversion
in byte versions of add/subtract.

- Solved issue with multiply modifier, which lead to white*white
not being white (was off by 1.0/255) due to wrong optimization
of division by 255 with shr by 8.

According to Ton this issue goes back to 2000!

This fixes #34811: Wrong result of add/subtract effects