Log footprints of images in active (#53)

In verbose mode, print the footprint of images counted for active. Add logs to indicate when the ambient and the active calculations start.

Do not deduplicate assets with a null sha (#46)

For fonts, we do not compute the sha for deduplication. There was a bug where all resources were put together under the same dedup key if they had a null sha. This change skips resources with a null sha from being deduped.