FMod - Attempts at foreground highlight

Somebody mentioned that it would be helpful to have a mode that highlights "front" foreground tiles, i.e. those that always appear in front of Keen and sprites. It's extremely easy to put a constant overlay on such tiles, but that's not very helpful in visualizing what parts will definitely appear in front. The obvious approach would be to get the image of the tile being marked up and make a translucent constant-color version of it, but in the current architecture, the function that creates the overlay image doesn't know which tile it's working with, only its Abiathar Tile Property ID. Multiple tiles can have the same properties, so the cache and the code paths will need to be reworked for this to happen. I do think it's worthwhile, so I'll need to figure something out.