Fix desync between fade out/in and level loading when game is lagging.

modules/game/effects.scm

@@ -1,5 +1,6 @@
 (define-module (game effects)
   #:use-module (dom canvas)
+  #:use-module (game scripts)
   #:use-module (game time)
   #:use-module (ice-9 match)
   #:use-module (math)
@@ -57,11 +58,21 @@
 (define game-width 320.0)
 (define game-height 240.0)
 
-(define (make-fade-out+in-effect duration)
+;; on-transition is a last-minute hack to make it possible to sync the
+;; loading of the next level with the moment when the screen is
+;; completely black.
+(define (make-fade-out+in-effect duration on-transition)
+  (define transitioned? #f)
+  (define (maybe-transition)
+    (unless transitioned?
+      (set! transitioned? #t)
+      (on-transition)))
   (define (draw context t)
     (if (< t 0.5)
         (set-global-alpha! context (* t 2.0))
-        (set-global-alpha! context (- 1.0 (* (- t 0.5) 2.0))))
+        (begin
+          (maybe-transition)
+          (set-global-alpha! context (- 1.0 (* (- t 0.5) 2.0)))))
     (set-fill-color! context "#000000")
     (fill-rect context 0.0 0.0 game-width game-height)
     (set-global-alpha! context 1.0))