[0.6.4] ALT overlay with empty running furnaces

[immibis]

If a furnace has no items in the input or output slot, but is processing an item, sometimes it will not show an overlay when ALT is active.

Screenshot:
Both furnaces are fed by inserters. The left one currently has no items in it, but is processing a piece of iron ore. The right one is processing a piece of iron ore and has two more pieces in its input slot. It also happens if the item is inserted manually.

This is easy to reproduce:

* build a (stone) furnace
* open the GUI
* press ALT
* move the GUI window out of the way so you can see the furnace and the GUI at the same time
* insert exactly one smeltable item
* notice there is no overlay showing the item being smelted

[kovarex]

Yes, that is the result of the changed logic (it will eat the input when starting the process).
We will fix it in 0.7