fix issue #1309 - secret doors in Garden rooms

Issue reported by elunna:  when a room gets converted into a theme
room with fill type Garden, its walls are changed to trees but any
secret doors in those walls are still displayed as regular walls.

This adds a new D_ARBOREAL flag for secret doors, used to force them
to be displayed as a tree instead of a wall.

Fixes #1309

src/detect.c

@@ -1588,7 +1588,7 @@ do_vicinity_map(
 void
 cvt_sdoor_to_door(struct rm *lev)
 {
-    int newmask = lev->doormask & ~WM_MASK;
+    int newmask = lev->doormask & ~(WM_MASK | D_ARBOREAL);
 
     if (Is_rogue_level(&u.uz)) {
         /* rogue didn't have doors, only doorways */

src/display.c

@@ -2289,6 +2289,13 @@ back_to_glyph(coordxy x, coordxy y)
     case CORR:
         idx = (ptr->waslit || flags.lit_corridor) ? S_litcorr : S_corr;
         break;
+    case SDOOR:
+        if ((ptr->doormask & D_ARBOREAL) != 0) {
+            idx = S_tree;
+            break;
+        }
+        FALLTHROUGH;
+        /*FALLTHRU*/
     case HWALL:
     case VWALL:
     case TLCORNER:
@@ -2300,7 +2307,6 @@ back_to_glyph(coordxy x, coordxy y)
     case TDWALL:
     case TLWALL:
     case TRWALL:
-    case SDOOR:
         idx = ptr->seenv ? wall_angle(ptr) : S_stone;
         break;
     case DOOR:
@@ -3579,6 +3585,10 @@ wall_angle(struct rm *lev)
         break;
 
     case SDOOR:
+        if ((lev->doormask & D_ARBOREAL) != 0) {
+            idx = S_tree;
+            break;
+        }
         if (lev->horizontal)
             goto horiz;
         FALLTHROUGH;

src/mkmaze.c

@@ -76,8 +76,16 @@ boolean
 set_levltyp(coordxy x, coordxy y, schar newtyp)
 {
     if (isok(x, y) && newtyp >= STONE && newtyp < MAX_TYPE) {
-        if (CAN_OVERWRITE_TERRAIN(levl[x][y].typ)) {
-            schar oldtyp = levl[x][y].typ;
+        schar oldtyp = levl[x][y].typ;
+
+        /* hack for secret doors in garden theme rooms */
+        if (oldtyp == SDOOR && newtyp == AIR) {
+            /* levl[][].typ stays SDOOR rather than change to AIR */
+            levl[x][y].doormask |= D_ARBOREAL;
+            return TRUE;
+        }
+
+        if (CAN_OVERWRITE_TERRAIN(oldtyp)) {
             /* typ==ICE || (typ==DRAWBRIDGE_UP && drawbridgemask==DB_ICE) */
             boolean was_ice = is_ice(x, y);