Allow flipping levels horizontally or vertically

When a special level is created, there's a chance it gets flipped
horizontally and/or vertically.

Add new level flags "noflip", "noflipx", and "noflipy" to prevent
flipping the level. Add a wiz-mode command #wizlevelflip to test
the flipping on current level - although this doesn't flip everything,
as level flipping is meant to happen during level creation.

dat/bigrm-2.lua

@@ -1,6 +1,6 @@
 
 des.level_init({ style = "solidfill", fg = " " });
-des.level_flags("mazelevel");
+des.level_flags("mazelevel", "noflip");
 
 des.map([[
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