Add a way to get the global z_index of a node.

[vvvvvvitor]

As the title says, there isn't much to it.

[NovaDC]

Seconded, this would be really useful and while it can be done in gdscript, this feels like a function that could be handled better by making it core (aka making the looping through parents a little faster)

[NovaDC]

For now though, the best solution is to use this type of function (also adapted from the code originally found in this post here):

static func get_absolute_z_index(target:CanvasItem) -> int:
   var z_index:int = 0
   while target and target is CanvasItem:
      z_index += target.z_index      
      if not target.z_as_relative:
         break
      target = target.get_parent()
   return z_index