🐛 Fix automatic pitch calculation in ScrewHole()

[nobodyinperson]

As ScrewThread() is called in ScrewHole() with scaled values for
outer_diam, the ThreadPitch() function in ScrewThread() outputs
non-matching values. Calculating the pitch independently fixes it.

• Fixes Automatic pitch calculation in ScrewHole is broken #1

[rcolyer]

On the commit about screw_resolution, this sort of usage of $fs (as a dynamic scope variable) in used libraries is not properly defined, and will not work in this manner in the next OpenSCAD release. It could be done by replacing it with a function call, but this is not a good idea for this library without also overriding the default $fs variable in some manner, because the default $fs is 2, and will cause almost every thread of interest to be a catastrophic failure under the default usage of the library. One possible resolution to this is to replace screw_resolution with a function that returns 0.2 if $fs is 2 or greater, otherwise it returns $fs. But this has a slight weakness as well because it has become a convention to set $fs to 0.4 on the most common 0.4mm extruder FDM printers, whereas threads are always vertical and there is some benefit to matching them to the layer height.

Perhaps the right answer is to change the top line to default_screw_resolution and then move screw_resolution into ScrewThread with a line like:
screw_resolution = is_undef($screw_resolution) ? default_screw_resolution : $screw_resolution;

That would still yield full user control, while not conflicting with established usage patterns.