Blender Git Loki

VSE: Implement sanity check for files with more channels than supported

This is a follow up to 8fecc2a8525467ee2fbbaae16ddbbc10b3050d46.

This makes sure future .blend files that have more channels than the
limit won't break Blender.

For LTS this happens silently, without warning the users.

This is part of https://developer.blender.org/D12645

Differential Revision: https://developer.blender.org/D12648