filter: add frame parallelizing filter wrapper

filter: add frame parallelizing filter wrapper

This wrapper can be used to frame parallelize simple video filters. By
simple, I mean there can be no temporal context that is shared from one
frame to the next.

Wrap unsharp and lapsharp filters.  unsharp required a small rework to
separate out temporary storage that is required when processing each
frame.  We now need to duplicate this storage for each thread.

Closes #759.