Implement flag that limits occlusion to only cross layer boundaries

[dwymark]

Hi LoicGoelfert, thanks for the excellent plug-in!

I found that I needed this feature for one of my sketch ideas, so I implemented it myself. I think it might be useful for others as well, so please take a look when you have a minute and let's talk about whether it makes sense to merge in my changes.

Here is the explanation that I added to the README for this new flag:

---

occult -a only performs occlusions across layers, ignoring occlusions that occur within a layer. As in occult -i, geometries in layers with a larger ID number are considered to be "on top" of geometries in layers with a smaller ID number. This option overrides -i.

• Without -a or -i flags

• With -i flag

• With -a flag

[LoicGoulefert]

Hi Daniel ! Thank you for this great PR. This is definitely a good feature to add.