app, libgimp, pdb: improve gimp_vectors_stroke_get_length() docs.

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.

app/pdb/vectors-cmds.c

@@ -1499,8 +1499,8 @@ register_vectors_procs (GimpPDB *pdb)
   gimp_procedure_add_argument (procedure,
                                g_param_spec_double ("precision",
                                                     "precision",
-                                                    "The precision used for the approximation",
-                                                    -G_MAXDOUBLE, G_MAXDOUBLE, 0,
+                                                    "The precision used for approximating straight portions of the stroke",
+                                                    0.0, G_MAXDOUBLE, 0.1,
                                                     GIMP_PARAM_READWRITE));
   gimp_procedure_add_return_value (procedure,
                                    g_param_spec_double ("length",

libgimp/gimpvectors_pdb.c

@@ -202,7 +202,7 @@ gimp_vectors_get_strokes (GimpVectors *vectors,
  * gimp_vectors_stroke_get_length:
  * @vectors: The vectors object.
  * @stroke_id: The stroke ID.
- * @precision: The precision used for the approximation.
+ * @precision: The precision used for approximating straight portions of the stroke.
  *
  * Measure the length of the given stroke.
  *

pdb/groups/vectors.pdb

@@ -179,8 +179,8 @@ sub vectors_stroke_get_length {
 	  desc => 'The vectors object' },
 	{ name => 'stroke_id', type => 'int32',
 	  desc => 'The stroke ID' },
-        { name => 'precision', type => 'float',
-          desc => 'The precision used for the approximation' }
+        { name => 'precision', type => '0.0 <= float', default => 0.1,
+          desc => 'The precision used for approximating straight portions of the stroke' }
     );
 
     @outargs = (