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/render/backend/renderworker.cpp

@@ -35,7 +35,8 @@ RenderWorker::RenderWorker(RenderBackend* parent) :
   parent_(parent),
   available_(true),
   audio_mode_is_preview_(false),
-  preview_cache_(nullptr)
+  preview_cache_(nullptr),
+  render_mode_(RenderMode::kOnline)
 {
 }