sfc/slicer.py: when flipping, also flip Z positions used for selecting levels

This led to toolpaths full of strange yet almost imperceptible errors
along the Z axis, just on the edge of what could be explained by
machining tolerances.

sfc/slicer.py

@@ -150,7 +150,10 @@ max_nz = 0
 inclined = 0
 for facet in mesh.Facets:
 	if abs(facet.Normal.z) >= 1 - epsilon:
-		z_raw[facet.Points[0][2]] = 1
+		z = facet.Points[0][2]
+		if flip:
+			z = bb.ZMax - z + bb.ZMin
+		z_raw[z] = 1
 	else:
 		nz = abs(facet.Normal.z)
 		if nz > epsilon: