Make mushrooms hide base terrain (#4185) - graphics change only

visual change to mushroom forest overlay that hides the base terrain, to make it reflect current actual behavior

data/core/terrain-graphics.cfg

@@ -439,6 +439,8 @@ C*,K*,X*,Q*,W*,Ai,M*,*^V*,*^B*,_off^_usr#enddef
 #
 
 # Flat base terrains
+# we want this base image to show up no matter what the underlying terrain is, because mushroom terrain is just odd for historical reasons
+{NEW:BASE                (*^Uf*)                                forest/mushroom-base LAYER=-319}
 
 # The first grass tile of each kind is the most standard and generic one, thus it appears more frequently to space out the rest
 {NEW:BASE_P              Gg                                     20                 grass/green}
@@ -841,6 +843,9 @@ C*,K*,X*,Q*,W*,Ai,M*,*^V*,*^B*,_off^_usr#enddef
 {NEW:TRANSITION            (Ms,Ha)          (W*,S*)                     -171               hills/snow-to-water}
 {NEW:TRANSITION            (Ms,Ha)          (!,Ha,Qx*,Mm,Ms,Md)         -172               hills/snow}
 
+{NEW:TRANSITION            (*^Uf*)          (!,M*,*^Uf*)                -184     forest/mushroom-base}
+{NEW:DISABLE_TRANSITION    (R*,G*,H*,U*,A*)	    (*^Uf*)	}
+{NEW:DISABLE_TRANSITION    (R*,G*,H*,U*,A*)	    (*^Uf*)	FLAG=inside}
 {NEW:TRANSITION            (Mm,Hh)          (!,M*,Hh,Ai,W*,S*)          -180               hills/regular}
 {NEW:TRANSITION            (Md,Hhd)         (!,M*,Hhd,Ai,W*,S*)         -183               hills/dry}
 {NEW:TRANSITION            Hd               (!,Hd,Qx*,W*)               -184               hills/desert}