Optimization opportunity using CAPortalLayer

[adamwulf]

https://medium.com/@avaidyam/caportallayer-7fc81e4fb385

What these properties allow you to do is “portal” a layer from any process whose context you know of - just like with `CAPluginLayer`. Just send the `CAContext.contextId` over the wire from a friendly process and it’ll render. Unlike `CALayerHost` et al. however, the originating layer will also render on-screen (unless `hidesSourceLayer` is true). How do you get the `sourceLayerRenderId` you might ask? Here’s the function prototype: `NSUInteger CALayerGetRenderId(CALayer *);` – it won’t be hard.