cache: reimplemented auto-caching

Implemented a smarter auto-cache that's context sensitive. Caching will
automatically pause when playback begins or values are being changed and resume
when inactive.

app/render/backend/renderbackend.cpp

@@ -204,6 +204,10 @@ void RenderBackend::UpdateInstance(RenderWorker *instance)
 
 void RenderBackend::Close()
 {
+  video_pool_.threads.clear();
+  audio_pool_.threads.clear();
+  hash_pool_.threads.clear();
+
   CancelQueue();
 
   video_pool_.Destroy();