The `precision` parameter in particular had no min/max, which meant we
could provide a forbidden parameter (e.g. a negative precision) which
would cause a core CRITICAL. We must forbid illegal values from PDB side
(hence outputting a normal plug-in error message, not a core bug).

Also improving a bit the description of this parameter as I was
wondering what precision was needed exactly to get a stroke length. This
is the precision for determining whether a portion of the stroke is
"straight enough" or if we want to break it into smaller pieces until we
get a straight portion.