Add critter silhouettes in combat mode

[darkf]

Fallout 2 has red/green/yellow silhouettes on critters determining their hostility, as seen here.

I'm not actually sure what yellow means (neutral?); green means friendly and red means hostile, though.

FO2 actually has a nice animation which kinds scanlines down, but for now we can just implement a solid outline color (probably doing it with a temporary canvas -- draw a solid outline color masked by the sprite scaled a little bit, then draw the sprite again over it.)

[darkf]

Done, 0158976.

Note that the idea of friendlies will have to be reworked, but that's another issue. (#72?)