offset miter limit to 3 - 801 828 836 851 875 - spikes and pimples

Was set to 10 to address 801. Setting to 3 has the same effect for 801,
and avoids spike artifacts that are likely causing "spike and pimple"
problems in the referenced issues.

lib/Slic3r/Geometry/Clipper.pm

@@ -21,7 +21,7 @@ sub offset {
     my ($polygons, $distance, $scale, $joinType, $miterLimit) = @_;
     $scale      ||= 100000;
     $joinType   //= JT_MITER;
-    $miterLimit //= 10;
+    $miterLimit //= 3;
     
     my $offsets = Math::Clipper::offset($polygons, $distance, $scale, $joinType, $miterLimit);
     return @$offsets;