Crashes were caused by setting `Strip::machine` above `MAX_CHANNELS`
value.
Instead of clamping this value at multiple places, setter function
`seq::strip_channel_set` was added that implements clamping.
This also allows for better handling of clamping if it was ever needed.
Function `strip_transform_handle_expand_to_fit` still sets machine value
directly and above `MAX_CHANNELS` as a hack. This could be avoided, but
it would require some refactoring.
Pull Request: https://projects.blender.org/blender/blender/pulls/136163
60de2103c0