Smoother fade-in for "stamina low" overlay/billboard

[keyspace]

Currently the "stamina low" overlay's alpha value is changed by the client as it's received from the server. This makes it somewhat "choppy" (once every 6 ticks/frames), and requires frequent updates (6 vs. 60) for the same reason.

If instead it transitioned from "previous value" to "current value" instead, according to the same curve as server-side, then its alpha can be updated client-side every tick/frame.