Combine ^gem and ^ghost-gem into one actor.

We still want the ghost gems to be collectable, even though they don't
do anything.

modules/game/actors.scm

@@ -12,7 +12,6 @@
             ^bomb
             ^brick
             ^gem
-            ^ghost-gem
             ^and-gate
             ^xor-gate
             ^or-gate
@@ -382,8 +381,9 @@
     (('describe) `(bomb ,position ,($ exploding?)))
     (('collide other offset grid-info) #f)))
 
-;; TODO: Combine with ghost gem.
-(define (^gem bcom x y)
+;; A gem that has already been collected previously will still appear
+;; in the level but it will be drawn differently.
+(define (^gem bcom x y previously-collected?)
   (define position (vector x y 1))
   (define picked-up? (spawn ^cell))
   (match-lambda*
@@ -396,27 +396,15 @@
     (('wire-state grid-info from from-x from-y) #f)
     (('update-wire-state grid-info neighbor-grid) #f)
     (('alive?) (not ($ picked-up?)))
-    (('describe) `(gem ,position))
+    (('describe)
+     (if previously-collected?
+         `(ghost-gem ,position)
+         `(gem ,position)))
     (('collide other offset grid-info)
      (when (eq? ($ other 'type) 'player)
        ($ picked-up? #t)
        ($ grid-info 'append-event `(pickup ,x ,y))))))
 
-(define (^ghost-gem bcom x y)
-  (define position (vector x y 1))
-  (match-lambda*
-    (('type) 'ghost-gem)
-    (('position) position)
-    (('tick grid-info) #f)
-    (('post-tick grid-info) #f)
-    (('enter obj grid-info) #f)
-    (('exit obj grid-info) #f)
-    (('wire-state grid-info from from-x from-y) #f)
-    (('update-wire-state grid-info neighbor-grid) #f)
-    (('alive?) #t)
-    (('describe) `(ghost-gem ,position))
-    (('collide other offset grid-info) #f)))
-
 (define (^gate bcom x y)
   (define position (vector x y 1))
   (define open? (spawn ^cell))

modules/game/level.scm

@@ -77,9 +77,7 @@
                         (8 (let ((target-x (bytevector-u8-ref objects (+ i 3)))
                                  (target-y (bytevector-u8-ref objects (+ i 4))))
                              (spawn ^floor-switch x y target-x target-y)))
-                        (9 (if collected-gem?
-                               (spawn ^ghost-gem x y)
-                               (spawn ^gem x y)))
+                        (9 (spawn ^gem x y collected-gem?))
                         (10 (spawn ^gate x y))
                         (11 (spawn ^and-gate x y
                                    (direction->symbol