fix #3288 "Extrude region node - keep original"

[satabol]

Sverchok 1.3.0-alpha, Blender 3.6.1, Windows 11

• fix Extrude region node - keep original #3288
• Create library for function separate_loose_mesh

Result is good:

File for tests: issue_3288.example.blend.zip

[satabol]

Hi @portnov! Can you check it? Some code are commented. I remove it later if you approve it.

[portnov]

If you think the problem is in Blender code, I'd suggest to file an issue against Blender, and add a link in code comment to that issue and write that you are doing a workaround for it.
Otherwise, no objections.

[satabol]

Hi @portnov.

this is copy of comment: #3288 (comment)

This is very interesting problem. I found some cases where it happens:

1. Your case with one face but there is more general case: with all extruded faces in mesh:
2. If all faces are extruded then property "Keep Original" is off has no effect:
   
   Some results are good. For example ("Keep original" is off), if 4-th face if excluded from extrude then result looks good:
   

but in other cases result is not good ("Keep original" is off), if 0 and 1 faces are excluded from extrude then result is not good:

So why this happens?
Lets look at blender source code for function "bmesh.ops.extrude_face_region":

So this is problem of Blender. Shame. What solution can be here? As you can see if exists a face in shape that is not extruded then result is good:

So... we cannot exclude some faces from extrude... but one can append a face to every shape then exclude it from extrude then remove new faces after extrude!

And result is good: