Fix cache key error

bin/dither/dither.rb

@@ -16,11 +16,9 @@ def cache_path_for(path, geometry)
   return unless CACHE_DIR
 
   source_path = "#{ROOT}/#{path}.enc"
-  stat = File.stat(source_path)
   cache_key = Digest::SHA256.hexdigest([
     path,
-    stat.size,
-    stat.mtime.to_f,
+    Digest::SHA256.file(source_path).hexdigest,
     geometry,
     DITHER,
     FORMAT,