Rethinking how to handle tiles that obscure the player and other entities

[dorkster]

Discussed in flareteam/flare-game#979

We currently draw an (optional) marker above entities that are obscured by tall tiles. I'm thinking it may be better to fade the obscuring tiles out to reveal what's behind them. Many other isometric games do something similar, and I think it'd be a better solution than what we're currently doing.