Added an option for enabling/disabling background processing

lib/Slic3r/GUI/Preferences.pm

@@ -44,6 +44,14 @@ sub new {
                 tooltip     => 'If this is enabled, Slic3r will auto-center objects around the configured print center.',
                 default     => $Slic3r::GUI::Settings->{_}{autocenter},
             },
+            {
+                opt_key     => 'background_processing',
+                type        => 'bool',
+                label       => 'Background processing',
+                tooltip     => 'If this is enabled, Slic3r will pre-process objects as soon as they\'re loaded in order to save time when exporting G-code.',
+                default     => $Slic3r::GUI::Settings->{_}{background_processing},
+                readonly    => !$Slic3r::have_threads,
+            },
         ],
         on_change => sub { $self->{values}{$_[0]} = $_[1] },
         label_width => 100,