Expose Iterations in the Compound Blur Effect

Add an Iterations parameter to the Compound Blur effect, similar to the existing parameter in Fast Box Blur.

Compound Blur can produce noticeably blocky results, suggesting that its internally used blur iteration count may be too low for certain use cases. Exposing this iteration count as a user-adjustable parameter would provide greater control over blur quality and allow users to achieve smoother, more refined results when needed, at the cost of additional processing time.

To ensure existing projects remain visually unchanged, the Iterations parameter should default to the number of iterations currently used internally by Compound Blur. This would preserve the existing appearance and behavior of the effect unless users explicitly change the setting.