fix ChildContainer.depth_list child removal when child's depth changed

[emgfc]

This is a follow up on #10362

I don't think this should actually be merged as is, just wanted to describe somewhere what's going on.

Ruffle has some sort of a leak in delayed DisplayObject child removing. When a child removing is delayed, that child object (and all of its children recursively) is assigned a negative depth and queued for removing in ChildContainer::queue_removal() func. Though there's a logic for updating that child element's position in depth_list (removing old ones, inserting new ones), it's not recursive, so nested children objects are having negative depths while being stored in depth_list with their previous positive depths as a keys. And this is the reason why we have lookup errors with that depth_list a bit later.

An easy and obvious fix for that would be implementing a recursive ChildContainer::remove_child_from_depth_list() and ChildContainer::insert_child_into_depth_list(). And yes, I did it, and everything just became worse when playing that slimy yellow sponge game.

I believe someone smarter than me could actually fix it the right way.