Part of #130975: split off overly large VSE effects.cc (almost 4000
lines) into separate source files. No behavior changes.
- Add / Subtract / Multiply all go into one source file,
- Blend Over / Blend Under / Over Drop / other blend modes all go
into one source file,
- Cross and Gamma Cross go into one source file,
- All others go into their own files.
Pull Request: https://projects.blender.org/blender/blender/pulls/132323