Add poison cloud glyph, fumaroles to fire plane.

When a gas cloud that deals damage is created, it uses
a poison cloud glyph instead of the cloud glyph.
(A bright green '#', or a bright-green recolor of the
cloud tile)

The plane of fire has random "stinking clouds", or
fumaroles, centered on lava pools.

Also make poison cloud glyph override lava, pool and
moat glyphs.

src/display.c

@@ -674,7 +674,12 @@ newsym(x,y)
 	 */
 	lev->waslit = (lev->lit!=0);	/* remember lit condition */
 
-	if (reg != NULL && ACCESSIBLE(lev->typ)) {
+	/* normal region shown only on accessible positions, but poison clouds
+	 * also shown above lava, pools and moats.
+	 */
+	if (reg != NULL && (ACCESSIBLE(lev->typ) ||
+		(reg->glyph == cmap_to_glyph(S_poisoncloud) &&
+		 (lev->typ == LAVAPOOL || lev->typ == POOL || lev->typ == MOAT)))) {
 	    show_region(reg,x,y);
 	    return;
 	}