Properly render negative score updates.

The old code displayed score updates using the "score-addition" class,
but only for positive updates; negative updates (possible only in
variants) didn't get that animation.
Merge branches 'AppearanceClasses', 'MergeRules', 'Sass345' and 'ScoreDecrements' into 2048-sqrt
2048-sqrt: Add "Nth root" tiles.