deck: flush surface upon exit

Fixes bug #180

lib/squib/card.rb

@@ -51,6 +51,10 @@ module Squib
       @cairo_context.restore
     end
 
+    def finish!
+      @cairo_surface.finish
+    end
+
     ########################
     ### BACKEND GRAPHICS ###
     ########################

lib/squib/deck.rb

@@ -73,6 +73,7 @@ module Squib
       if block_given?
         instance_eval(&block) # here we go. wheeeee!
       end
+      @cards.each { |c| c.finish! }
     end
 
     # Directly accesses the array of cards in the deck