sequence: made divider a sequence parameter rather than a viewer parameter

Doing this makes the preview resolution an explicit project setting rather
than a temporary UI setting. This will allow more consistency when caching.

app/task/export/export.cpp

@@ -33,6 +33,9 @@ ExportTask::ExportTask(ViewerOutput* viewer_node,
   params_(params)
 {
   SetTitle(tr("Exporting \"%1\"").arg(viewer_node->media_name()));
+
+  // Render highest quality
+  backend()->SetRenderMode(RenderMode::kOnline);
 }
 
 bool ExportTask::Run()