Update read marker only when you actually scroll up

This prevents some terrible user experience, where you continue where you left off and then quit Tokodon without reading anything. It would previous set the read marker to a seemingly random place, because it didn't correspond to where you actually stopped reading.

Now that's changed, and Tokodon will only update the read marker if you start reading said post by reusing the same logic for stopping videos from playing in the background when you scroll.