Crater updates and other minor terrain fixes (#6492)

* fix bad Swamp-Icy_Cobble (Ss-Rra) transition

* uncouple crater from desert sands

* different craters (^Dc) for sand, dirt, and ice

* fix some dull snow on Koa and Coa

data/core/terrain-graphics.cfg

@@ -521,6 +521,8 @@ C*,K*,X*,Q*,W*,Ai,M*,*^V*,*^B*,_off^_usr#enddef
 
 {NEW:BASE                Rd                                                        flat/desert-road}
 
+{NEW:BASE                (R*^Dc,G*^Dc)                                             flat/crater FLAG=crater}
+{NEW:BASE                A*^Dc                                                     frozen/crater FLAG=crater LAYER=-999}
 {NEW:BASE                *^Dc                                                      sand/crater FLAG=crater}
 
 {NEW:BASE                Hd                                                        hills/desert}
@@ -930,7 +932,7 @@ C*,K*,X*,Q*,W*,Ai,M*,*^V*,*^B*,_off^_usr#enddef
 {NEW:TRANSITION            Ai,W*,S*         Ur                          -223               cave/floor FLAG=inside}
 {NEW:TRANSITION            Urb              (!,Urb)                     -224               cave/flagstones-dark}
 
-{NEW:TRANSITION            Ss               (!,I*,Ss,H*,M*,A*,Chs,K*,Q*)   -230               swamp/water}
+{NEW:TRANSITION            Ss               (!,I*,Ss,H*,M*,A*,Chs,K*,Q*,Rra)   -230               swamp/water}
 
 #{NEW:TRANSITION            Iwr              G*,R*,W*,S*,D*,A*,Ur        -230               interior/wood-regular}
 # need to move this up higher to get over chasm walls

data/core/terrain.cfg

@@ -353,9 +353,10 @@ Most units receive 20 to 40% defense in sand."
     symbol_image=sand/crater
     id=crater
     name= _ "Crater"
-    string=Dd^Dc
+    string=^Dc
     default_base=Dd
     aliasof=Dt
+    mvt_alias=-,_bas,Dt
     editor_group=desert
 [/terrain_type]