Feature: Compute first N coarsest levels in a single worker during AWS renders

[aparrapo]

The current behavior and new feature

At any level of the pyramid a worker needs to download the color level for a frame from S3, perform depth estimation, and then pack and send the results back.

At the coarsest levels we end up spending more time downloading and uploading data than actually processing the frames. We can speed up the process by letting workers do the coarsest N levels (e.g. levels 9 to 4) at once.

Note that this means not being able to run the temporal filter after each of these levels, since that stage is also distributed, but it may be a good trade-off.

Expected use cases

Faster renders at the expense of worse temporal smoothing, but may be not noticeable.