Performance improvements in reloading the 3D scene.

Now the 3D scene loading is postponed until the page is visible.

lib/Slic3r/GUI/Plater/3DPreview.pm

@@ -131,10 +131,16 @@ sub new {
 }
 
 sub reload_print {
-    my ($self) = @_;
+    my ($self, $force) = @_;
     
     $self->canvas->reset_objects;
     $self->_loaded(0);
+
+    if (! $self->IsShown && ! $force) {
+        $self->{reload_delayed} = 1;
+        return;
+    }
+
     $self->load_print;
 }
 
@@ -289,4 +295,10 @@ sub set_number_extruders {
     }
 }
 
+# Called by the Platter wxNotebook when this page is activated.
+sub OnActivate {
+    my ($self) = @_;
+    $self->reload_scene(1) if ($self->{reload_delayed});
+}
+
 1;