Update export to video implementation

[tginart]

Implement a cautious multi-pass video export pipeline that renders segments and tracks separately before final composition, falling back to the original monolithic approach on failure.

The monolithic FFmpeg -filter_complex can become extremely complex and difficult to debug or scale for intricate compositions. This multi-pass approach breaks down the composition into smaller, more manageable FFmpeg commands, rendering individual segments and then tracks with alpha channels, before a final overlay. This improves robustness and provides a fallback mechanism to the proven monolithic path if the new pipeline encounters issues.

---

[cursor]

Cursor Agent can help with this pull request. Just @cursor in comments and I'll start working on changes in this branch.
_{Learn more about Cursor Agents}