jhatwich/layer comps+size

To use this feature add a layer comp and name it something like "overview.png" or "200% overview.png"
And if that's the case, then it seems like this can be handled just like it's handled in the `AssetManager`, but in whatever module from your plugin is driving the `RenderManager`. In any case, I don't want to this to be handled at two different levels of abstraction. Either all the management should go into the `RenderManager`, or clients of the `RenderManager` should be responsible for this management, as it is now.