Avoid premapping outside Sokoban map to prevent showing stone glyphs

If user has changed the stone glyph to something other than a space
(or uses a tileset), Sokoban levels showed the unreachable stone outside
the map area.  Prevent marking those areas as seen, so the stone
glyphs aren't shown.

src/detect.c

@@ -38,6 +38,7 @@ staticfn void foundone(coordxy, coordxy, int);
 staticfn void findone(coordxy, coordxy, genericptr_t);
 staticfn void openone(coordxy, coordxy, genericptr_t);
 staticfn int mfind0(struct monst *, boolean);
+staticfn boolean skip_premap_detect(coordxy, coordxy);
 staticfn int reveal_terrain_getglyph(coordxy, coordxy, unsigned, int,
                                      unsigned);
 
@@ -2117,6 +2118,16 @@ warnreveal(void)
         }
 }
 
+/* skip premap detection of areas outside Sokoban map */
+staticfn boolean
+skip_premap_detect(coordxy x, coordxy y)
+{
+    if ((levl[x][y].typ == STONE)
+        && (levl[x][y].wall_info & (W_NONDIGGABLE | W_NONPASSWALL)) != 0)
+        return TRUE;
+    return FALSE;
+}
+
 /* Pre-map (the sokoban) levels */
 void
 premap_detect(void)
@@ -2128,6 +2139,8 @@ premap_detect(void)
     /* Map the background and boulders */
     for (x = 1; x < COLNO; x++)
         for (y = 0; y < ROWNO; y++) {
+            if (skip_premap_detect(x, y))
+                continue;
             levl[x][y].seenv = SVALL;
             levl[x][y].waslit = TRUE;
             if (levl[x][y].typ == SDOOR)