v0.18.0: Release 0.18.0: presentations from the same scenes.

Presentations from the same scenes.

Added

• slide_break().await marks a stop. A render plays straight through it;
  oxframe present (CLI 0.18.0) pauses there until the next key press.
  The last frame of every scene is a stop too.
• notes("…") adds speaker notes for the part that ends at the next stop.
• render --slides talk.pdf writes a PDF with the frame of every stop,
  one per page, without ffmpeg.
• The timeline sent to the editor lists the stops.
• The slides example.

Fixed

• Rendering with --subtitles from oxframe render now works: the CLI
  passes the flag to the project (CLI 0.18.0).