Add 2 variables to switch off rendering of main parts / connectors

[akoehn]

$render_connectors and $render_main can be set to false to only render
connectors or only main parts to have two objects in the slicer.
this way, one can e.g. easily have thicker infill for the connectors.

I also have some scripts to automate rendering to STL from puzzlecad files which use this feature. Don't know where they (or additional documentation on how to use this feature to create sturdier connectors) should best be put.

I have to apply the change in #37 to be able to run puzzlecad on my machine; it's not in this PR though.

[tomburns]

@akoehn I wonder if this is superseded somewhat by the new feature in Prusaslicer that automatically uses solid infill for narrow areas?

[akoehn]

Could be; I have not used that feature yet (time ...).  Did you try it?

[tomburns]

I have - I will confess that I was often too lazy to use this model splitting approach, but I've configured Prusaslicer now to automatically fill in areas the size of ~ the male connectors or smaller and the results have been pretty satisfactory. This can get merged anyway for other slicers of course, but I thought it was worth a shot!

[akoehn]

Tom Burns writes:

I've configured Prusaslicer now to automatically fill in areas the size of ~ the male connectors or smaller and the results have been pretty satisfactory.

I always viewed this PR to be a kind of a hack and I use prusaslicer, so I don’t care about merging this that much anymore since reading those cool news :-)