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

git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@55924 954f8c5b-7b00-dc11-b283-0030488c597c