Reverted change from #241
The change of moving to the next available position when a space was partially blocked has caused too many issues, so now going back to searching vertically for the next available place. I will have to look at other solutions for #241.
NOTE: I have tried to make it a bit more efficient by starting at the current layer height instead of searching the whole available height, so that does mean it probably won't stack very well when you load items with mixed heights in layer mode.