Preview playback

PlaybackEngine (src/playback) shows the timeline in real time.

• One worker thread owns every MLT object used for preview: an MLT profile at preview resolution (the project's aspect ratio, at most the monitor's size), a TimelineGraphBuilder and the current tractor. The GUI posts commands (set project, play, pause, seek, step, preview size, volume) to a queue.
• Snapshots, not shared state. setProject copies the project. Edits are debounced by 30 ms in the UI and then rebuilt into a new tractor; source producers are cached per asset and track, so rebuilding does not reopen files.
• Clock. With an audio device (SDL 2 in push mode), the engine keeps about 120 ms of audio queued and shows each picture when its audio starts playing. Without one, frames are paced by a monotonic clock; if rendering falls far behind, the clock resets instead of racing to catch up.
• No backlog. Seeks are coalesced (only the latest pending seek is kept) and the UI receives only the most recent frame through a single pending notification, so scrubbing or a slow repaint cannot pile up work.
• Display. FrameView uploads the frame as a texture through the scene graph and draws it aspect-fitted. It works with every Qt Quick backend, including the software renderer.

Offline media appear as dark red placeholder frames during preview and are listed as warnings.

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