impr: precision for rendering

- Better precision for circle resolution:
  - See https://en.wikibooks.org/wiki/OpenSCAD_User_Manual/Other_Language_Features#Circle_resolution:_$fa,_$fs,_and_$fn

serre-tringle.scad

@@ -10,6 +10,9 @@ $radial_indent = 1;
 $slack = 0.1;
 $mini_slack = 0.001;
 
+// rendering precision
+$fn = $preview ? 32 : 64;
+
 // base
 difference() {
     cylinder($length_base, $diameter_z / 2, $diameter_z / 2);