Only draw the slices in toolpath preview for the current selected layer

lib/Slic3r/GUI/Plater/2DToolpaths.pm

@@ -242,8 +242,10 @@ sub Render {
     foreach my $layer (@{$self->layers}) {
         my $object = $layer->object;
         
+        # only draw the slice for the current layer
+        next unless abs($layer->print_z - $self->z) < epsilon;
+        
         # draw slice contour
-        {
         glLineWidth(1);
         foreach my $copy (@{ $object->_shifted_copies }) {
             glPushMatrix();
@@ -275,7 +277,6 @@ sub Render {
             glPopMatrix();
         }
     }
-    }
     
     my $skirt_drawn = 0;
     my $brim_drawn = 0;