Refine bomb logic

This commit is the first of a series introducing hidden logic through
non-obvious commits. I'm mentioning it in the commit messages in the hopes no
one reads these so I can sneak it in without leaving no record at all ;)

modules/game/level.scm

@@ -67,7 +67,12 @@
           (when (< x width)
             (let* ((i (+ (* y width) x))
                    (pos (vec2 (* x tile-width) (* y tile-height)))
-                   (id (bytevector-u16-native-ref background (* i 2)))
+                   (id (case (bytevector-u16-native-ref background (* i 2))
+                         ((120) 81)
+                         ((121) 82)
+                         ((122) 85)
+                         ((123) 105)
+                         (else => (lambda (v) v))))
                    (tile (make-level-tile pos id)))
               (vector-set! background* i tile))
             (x-loop (1+ x))))