Fix black frames in heavy compose renders

A compose timeline with several video layers could go wrong partway through. A graded base clip with a picture-in-picture and a blended overlay on top was enough to trigger it at 1080p. The render either stalled, failed, or finished with every frame after a certain point black. In our test case the picture was correct for the first 32.8 seconds of a 59 second video and black after that. The last case is the worst one, because the job still reported complete.

The cause was on our side. Each render runs under a 14 GiB memory limit, and that limit counts reserved address space, not memory in use. A layered render reserved enough to reach it while using about 2 GB at 1080p and about 8 GB at 4K. Once it did, the colour and scaling steps failed on every frame and the render carried on without them.

Renders now stay well inside the limit. We re-ran the failing timelines at 1080p and 4K, and every one completes with output that is byte-identical to a render with no limit at all. A light 1080p timeline takes the same time as before, within normal run-to-run variation.

• Timelines with one video layer, text, shapes and music were not affected.
• Nothing changes in the request. No new fields, no new limits.
• If a compose output of yours went black partway through, run the job again.

See the compose job reference for the timeline format, and the earlier round of compose render fixes for blend modes, Ken Burns and clip volume.