nodes: implemented infrastructure for caching any node

Not accessible through UI yet, but ported Footage node to it
(previously we used a hack to ensure Footage textures were
cached, now it's a formal infrastructure).

app/node/node.cpp

@@ -48,7 +48,8 @@ Node::Node(bool create_default_output) :
   override_color_(-1),
   last_change_time_(0),
   folder_(nullptr),
-  operation_stack_(0)
+  operation_stack_(0),
+  cache_result_(false)
 {
   if (create_default_output) {
     AddOutput();