cache: mild auto-cache overhaul

Shifted from CacheTask to functionality built into RenderBackend. It was
a lot easier to control behavior this way without having to juggle a ton
of threads and race conditions.

Could likely be multithreaded further.

app/panel/viewer/viewerbase.cpp

@@ -90,6 +90,16 @@ void ViewerPanelBase::SetGizmos(Node *node)
   static_cast<ViewerWidget*>(GetTimeBasedWidget())->SetGizmos(node);
 }
 
+void ViewerPanelBase::CacheEntireSequence()
+{
+  static_cast<ViewerWidget*>(GetTimeBasedWidget())->CacheEntireSequence();
+}
+
+void ViewerPanelBase::CacheSequenceInOut()
+{
+  static_cast<ViewerWidget*>(GetTimeBasedWidget())->CacheSequenceInOut();
+}
+
 void ViewerPanelBase::CreateScopePanel(ScopePanel::Type type)
 {
   ViewerWidget* vw = static_cast<ViewerWidget*>(GetTimeBasedWidget());