Fix #34782: Video Editor - Substract after transform fails

Couple of precision issues here:

- Interpolation was rounding trunkcating colors, and because
  of some precision issues value 254.999 became 254 leading
  to troubles later.

  Now color interpolaiton will do rounding to nearest int.

- Subtract was setting channels to something negative which
  confused color management (especially negative alpha),

  Now subtract effect will clamp channels to 0 from bottom
  and also do some tricks to prevent small alpha which
  could also confuse color management.

git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@55644 954f8c5b-7b00-dc11-b283-0030488c597c