Feature: Clone tool opacity separate from source opacity and brush opacity

[canadaduane]

Feature request: Clone tool opacity, similar to Pen tool opacity. I like to use the Clone tool for texture touch-ups, but it needs to combine with the destination rather than overwrite it with full source layer opacity.

Here is a screenshot of the desired behavior (taken from Krita):

https://github.com/bgrabitmap/lazpaint/assets/129/826b800f-023f-4111-96d6-42fcd0ff5354