The decoder's output size was frozen to the document size at import, and export
reused that decoder — so exporting above document res upscaled the video (real
source detail discarded) and a document resize never re-targeted the decode.
Decode size is now chosen per get_frame call: VideoDecoder::get_frame and
VideoManager::get_frame take a target (w, h), capped to native (never upscale),
with the swscale context and frame caches keyed on the output size so preview
(preview res) and an in-progress export (export res) don't collide. The renderer
derives the target from the document->output base_transform, so export decodes
at export res (full detail) and the canvas at preview res. Thumbnails/asset
library pass small targets.
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