grab bag of incomplete features/fixes

[andykais]

• optionally round background color
• 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
• 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  • Super important, now we can tell you if youre using outdated/ill formatted fields.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
• new template format
  • new timeline format
  • stop_relative_to_output?
• size/crop/layout all use the same data interface
• draw captions via deno-canvas, and drop imagemagick requirement
  • requires context.measureText api, which we need to implement here [feature request] loading custom font support DjDeveloperr/deno-canvas#17 note that we replaced this with the native skia ffi canvas implementation
• zoompan
• duration variable expressions