jhatwich/layer comps+size

To use this feature add a layer comp and name it something like "overview.png" or "200% overview.png"
This is added in case there are multiple clients.  We have AssetManager requesting renders and getting the original promise.  We are also hosting a web server whereby additional requests can come in while rendering is happening.  This API allows us to attach to a render that is already requested instead of blindly throwing a render request into the queue.