Fix/Add inventory_overlay for nodes without inventory_image

[Desour]

Closes #8430.
There might be many mistakes.

How to test this:

minetest.register_node("test_invcube:node", {
	description = "inventorycube node",
	tiles = {"unknown_item.png"},
	inventory_overlay = "camera_btn.png",
	groups = {oddly_breakable_by_hand = 3},
})

Here's a sceenshot:

As you can see, the rotating of the node also works (inventory_items_animations).

If the inventory_overlay image is not quadratic, it is squashed in one direction (try with progress bar texture).

[ClobberXD]

Awesome. Will test today.

[ClobberXD]

It works! (Note that I've only tested it with the code linked to in #8430)