Depsgraph: clear update flags when skipping through no-op nodes.

As an optimization, dependency graph evaluation skips through
no-op nodes at the scheduling stage. However, that leaves update
flags enabled on the node, and the most problematic one is the
USER_MODIFIED flag, which get flushed to children every time
the no-op node is tagged.

This in turn can cause simulation caches downstream to be
permanently stuck in an outdated state until the depsgraph
is rebuilt and the stuck flag cleared out.

Differential Revision: https://developer.blender.org/D16868

source/blender/depsgraph/intern/node/deg_node_operation.h

@@ -224,6 +224,10 @@ enum OperationFlag {
 
   /* Set of flags which gets flushed along the relations. */
   DEPSOP_FLAG_FLUSH = (DEPSOP_FLAG_USER_MODIFIED),
+
+  /* Set of flags which get cleared upon evaluation. */
+  DEPSOP_FLAG_CLEAR_ON_EVAL = (DEPSOP_FLAG_DIRECTLY_MODIFIED | DEPSOP_FLAG_NEEDS_UPDATE |
+                               DEPSOP_FLAG_USER_MODIFIED),
 };
 
 /* Atomic Operation - Base type for all operations */