grab bag of incomplete features/fixes

[andykais]

• [x] optionally round background color
• [x] ignore transitions on preview
  • we could make previews derrive from +x seconds into a transition, rather than always shifting everything exactly to the preview, but that will be a bit of a PITA
• [x] zod parser for template input
  • Super important, now we can tell you if youre using outdated/ill formatted fields.
  • we dont yet know if were using a typescript interface as the source of truth, or a zod validator
    • a zod validator has poor support for documentation generation
    • a typescript interface is hard to enforce on a zod validator
• [x] new template format
  • [x] new timeline format
  • [ ] stop_relative_to_output?
• [x] size/crop/layout all use the same data interface
• [x] draw captions via deno-canvas, and drop imagemagick requirement
  • [ ] ~requires context.measureText api, which we need to implement here https://github.com/DjDeveloperr/deno-canvas/issues/17~ note that we replaced this with the native skia ffi canvas implementation
• [ ] zoompan
• [ ] duration variable expressions