nodes: minor overhaul to functionality

The nodes now have more control over how their accelerated shaders/sample
functions are run, as well as how items are popped off the value tables.
This allows for various optimizations that we didn't have access to before.

app/widget/viewer/viewerdisplay.cpp

@@ -241,7 +241,7 @@ void ViewerDisplayWidget::paintGL()
 
   // Draw gizmos if we have any
   if (gizmos_) {
-    GizmoTraverser gt;
+    GizmoTraverser gt(QSize(gizmo_params_.width(), gizmo_params_.height()));
 
     rational node_time = GetGizmoTime();