Takes packed frames off the renderer and puts them on the panel.

BB.NSK.Display.Viewport renders into this process, which receives {:emerge_skia_frame, %VideoInterop.Frame{}} messages carrying a frame already packed to the panel's format. Nothing here converts pixels.

A refresh takes seconds and the renderer can produce frames faster than that, so drawing happens in a spawned process and only the latest frame waits: a burst of renders costs one refresh of the last one rather than a queue of stale ones. That matters because the robot's safety state can change several times inside a single refresh.

Emerge sends nothing at all when a screen renders to an unchanged tree, which is most of the deduplication and it happens before any of this. What is left here catches the rarer case of two different trees packing to the same bytes, such as a change that lands where the layout clips it.

That upstream silence is also a trap, and configure/3 works around it. A full refresh is asked for when the panel needs clearing rather than when the picture changed — it is what removes the ghosting partial refreshes leave — so it is exactly the case where the tree is most likely to be identical and no frame will arrive. Asking for one therefore arms a timer, and if nothing has been rendered by the time it fires the retained frame is redrawn instead. Without that, a panel showing an unchanging screen would never be cleaned up, and BB.NSK.Display.Controller.refresh/1 would silently do nothing.

On the host there is no panel, so the draw function is a no-op and the sink is a place to read the last frame from — which is how a screen is previewed without hardware.