scenes/mainmenu: make object menu take screen size from screen object

imperium-porcorum.love/scenes/debugmenu/menuobject.lua

@@ -4,11 +4,12 @@ local MenuObject = ListBox:extend()
 local Widgets = require "scenes.debugmenu.widgets"
 
 function MenuObject:new(scene, name)
+  local screenwidth, screenheight = core.screen:getDimensions()
   local height, width, x, y
-  height = 240 - 88 - 24
+  height = screenheight - (88 + 24)
   width  = 240
-  x = 424/2 - width/2
-  y = 240/2 - height/2 - 8
+  x = screenwidth/2   - width/2
+  y = screenheight/2  - height/2 - 8
 
   MenuObject.super.new(self, scene.menusystem, name, x, y, width, height, 5)
   self.scene = scene