This construct "current_x = x or current_x" was not correctly handling
cases where x could be 0 and current_x None, and current_x still be None
after the statement while it should have been 0.

This reverts commit 1877d2cc.

Conflicts:
	printrun/gcoder.py

In case we find two gcodes closer than 0.01mm in Z, we estimate the layer
height from the first layers and group gcodes based on this layer height. The
logic seems fine and the code seems to work, but it's nontrivial and I'm a bit
worried that it might break things.