Lowlights solved tiles in countdownMode

[misterperson]

I wanted to generalize this to outside of countdown mode, too, but I added a lowlight effect to solved tiles, an effect Tametsi achieves even outside of countdown mode.

[misterperson]

The conditional that checks if a tile is solved can be simplified to checking if all neighbors of a tile are revealed or flagged instead of looping over 3 times to do some math