item_entity: Cache collisionbox for use in on_step

I don't have absolute numbers but if calls to get_properties() take up
30%+ of on_step() execution time that's bound to matter.

builtin/game/item_entity.lua

@@ -59,17 +59,20 @@ core.register_entity(":__builtin:item", {
  math.floor(def.light_source / 2 + 0.5)
 
  local size_bias = 1e-3 * math.random() -- small random bias to counter Z-fighting
+ local c = {-size, -size, -size, size, size, size}
  self.object:set_properties({
  is_visible = true,
  visual = "wielditem",
  textures = {itemname},
  visual_size = {x = size + size_bias, y = size + size_bias},
- collisionbox = {-size, -size, -size, size, size, size},
+ collisionbox = c,
  automatic_rotate = math.pi * 0.5 * 0.2 / size,
  wield_item = self.itemstring,
  glow = glow,
  })
 
+ -- cache for usage in on_step
+ self._collisionbox = c
  end,
 
  get_staticdata = function(self)
@@ -94,6 +97,7 @@ core.register_entity(":__builtin:item", {
  self.object:set_armor_groups({immortal = 1})
  self.object:set_velocity({x = 0, y = 2, z = 0})
  self.object:set_acceleration({x = 0, y = -gravity, z = 0})
+ self._collisionbox = self.initial_properties.collisionbox
  self:set_item()
  end,
 
@@ -164,7 +168,7 @@ core.register_entity(":__builtin:item", {
  local pos = self.object:get_pos()
  local node = core.get_node_or_nil({
  x = pos.x,
- y = pos.y + self.object:get_properties().collisionbox[2] - 0.05,
+ y = pos.y + self._collisionbox[2] - 0.05,
  z = pos.z
  })
  -- Delete in 'ignore' nodes
@@ -177,7 +181,7 @@ core.register_entity(":__builtin:item", {
  if self.force_out then
  -- This code runs after the entity got a push from the is_stuck code.
  -- It makes sure the entity is entirely outside the solid node
- local c = self.object:get_properties().collisionbox
+ local c = self._collisionbox
  local s = self.force_out_start
  local f = self.force_out
  local ok = (f.x > 0 and pos.x + c[1] > s.x + 0.5) or