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/render/rendermanager.cpp

@@ -189,7 +189,7 @@ RenderTicketPtr RenderManager::SaveFrameToCache(FrameHashCache *cache, FramePtr
   // Create ticket
   RenderTicketPtr ticket = std::make_shared<RenderTicket>();
 
-  ticket->setProperty("cache", Node::PtrToValue(cache));
+  ticket->setProperty("cache", cache->GetCacheDirectory());
   ticket->setProperty("frame", QVariant::fromValue(frame));
   ticket->setProperty("hash", hash);
   ticket->setProperty("type", kTypeVideoDownload);