Implement workUnitToThreadRatio

[dzenanz]

Add workUnitToThreadRatio (work units = which multiple of threads) per filter, and set this to 1 or another low number for filters which have big overhead for splitting the image into pieces. Benchmark to determine best default value.

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.

[dzenanz]

It would still be good to do this.

[stale]

This issue has been automatically marked as stale because it has not had recent activity. Thank you for your contributions.

[dzenanz]

We should still do this.