Added two variants of 2x2 snow mountains

Some color tweaks to the single tile snow mountains

Reduced probability for 2x2 variants to offset for still missing 2x4
variants

data/core/terrain-graphics.cfg

@@ -101,8 +101,8 @@
 #{MOUNTAINS_1x3_NW_SE     Ms                                     20        overlay  snow-mountain_range1}
 #{MOUNTAINS_1x3_SW_NE     Ms                                     20        overlay  snow-mountain_range2}
 
-#{MOUNTAINS_2x2           Ms                                     40        overlay  snow-mountain5}
-#{MOUNTAINS_2x2           Ms                                     30        overlay  snow-mountain6}
+{MOUNTAINS_2x2           Ms                                     15        overlay  snow-mountain5}
+{MOUNTAINS_2x2           Ms                                     25        overlay  snow-mountain6}
 
 {MOUNTAIN_SINGLE         Ms                                     33        overlay  snow-mountains3}
 {MOUNTAIN_SINGLE         Ms                                     50        overlay  snow-mountains2}