fixes jitters by using signals/slots to pass image buffers around

Updating values rapidly would cause strange jitters as a
byproduct of the viewer trying to update from the renderer while
it was still working. Rather than the viewer trying to access the
the renderer, we now send textures in the initial update signal
to keep everything synchronized.

app/render/backend/videorenderworker.cpp

@@ -29,9 +29,9 @@ void VideoRenderWorker::RenderInternal(const NodeDependency& path)
     emit HashAlreadyExists(path, hash);
   } else if (frame_cache_->TryCache(hash)) {
     // This hash is available for us to cache, start traversing graph
-    RenderAsSibling(path);
+    QVariant value = RenderAsSibling(path);
 
-    emit CompletedFrame(path, hash);
+    emit CompletedFrame(path, hash, value);
   } else {
     // Another thread must be caching this already, nothing to be done
     emit HashAlreadyBeingCached();
@@ -115,7 +115,7 @@ void VideoRenderWorker::CloseInternal()
   download_buffer_.clear();
 }
 
-void VideoRenderWorker::RenderAsSibling(NodeDependency dep)
+QVariant VideoRenderWorker::RenderAsSibling(NodeDependency dep)
 {
   NodeOutput* output = dep.node();
   Node* original_node = output->parent();
@@ -158,6 +158,8 @@ void VideoRenderWorker::RenderAsSibling(NodeDependency dep)
 
   // End this working state
   working_--;
+
+  return value;
 }
 
 void VideoRenderWorker::Download(NodeDependency dep, QByteArray hash, QVariant texture, QString filename)