Preview workers that start together miss the layer cache together, and each kept its own copy
of every layer; on the GPU each copy was uploaded and packed into its own Classic 3D atlas, which
ran the device out of memory on a cold start. LayerCache::insert now keeps the first buffer of a
key and returns it, and the renderer goes on with the returned buffer, so the workers share one.
Renders don't wait for each other: on the shared frame pool a blocking wait could end up beneath
the work it waits for (tried, it stalled playback for the wait's timeout).
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
