When I was learning VSE code, MAXSEQ constant (a preprocessor define!) was
confusing. It makes it sound like it is "max sequences", but it is actually
"max channels". So rename it to SEQ_MAX_CHANNELS and make it C++ constexpr int
instead of preprocessor macro.
Pull Request: https://projects.blender.org/blender/blender/pulls/128024
ecce5f45ac