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/project/footage/footage.cpp

@@ -48,6 +48,8 @@ Footage::Footage(const QString &filename) :
   Clear();
 
   set_filename(filename);
+
+  SetCacheTextures(true);
 }
 
 void Footage::Retranslate()