C&C mod snow brightness fix

Reduces brightness of snow a little, gives more contrast to darker patches.

There recently was a complaint somewhere (forgot where it was) that the snow of the cnc mod is too bright. This was likely a result of the conversion, since C&C and RA use different palettes.

I figured the easiest way to fix this was editing snow.pal. The C&C snow terrain mostly uses color 239 for regular snow and color 192 for darker patches, so I reduced the brightness of color 239 by two steps, and decreased the brightness of color 192 by 4 steps (except for the blue value, which got decreased by only 1 step).