Allow several achievements to be displayed at the same time

like in steam
the change to popup_animation is not thoroughly tested

(cherry picked from commit f714e482f6)

game/scripts/interface/achievements.rpy

@@ -60,7 +60,7 @@ init python:
 
     class Achievements(object):
         """
-        Useless class, to be rolled out.
+        Useless class, can't be rolled out because of pickle and save compatibility.
         """
 
         @staticmethod
@@ -84,7 +84,7 @@ init python:
 
                 if not silent:
                     renpy.play('sounds/achievement.ogg')
-                    renpy.show_screen("achievement_window", msg=achievements_db[id].title, title="Achievement unlocked!", icon=achievements_db[id].icon)
+                    __popup_stack.append(id)
 
         @staticmethod
         def lock(self, id):
@@ -118,6 +118,21 @@ default achievements = Achievements()
 
 ###
 
+init python:
+    # intentionaly not a define nor a default
+    __popup_stack = []
+
+    config.after_default_callbacks.append(__popup_stack.clear)
+    config.always_shown_screens.append("achievement_main")
+
+screen achievement_main():
+    layer "interface"
+
+    vbox:
+        for id index id in __popup_stack[:3]:
+            use achievement_window(msg=achievements_db[id].title, title="Achievement unlocked!", icon=achievements_db[id].icon)
+            timer 6 action Function(__popup_stack.remove, id)
+
 label popup(msg="", title="", icon=None, xpos=0, ypos=60, sound=True, soundfile='sounds/achievement.ogg'):
     if sound:
         play sound soundfile

game/scripts/interface/popup.rpy

@@ -1,7 +1,4 @@
 transform popup_animation(time=4.0, xx=-200):
-    on start:
-        xoffset xx
-    on show:
     xoffset xx
     linear 0.5 xoffset absolute(0)
     pause time