Select opposite start-vertex for backwards paths.
Select opposite start-vertex for backwards paths.

The 'backwards' in alternating path mode ('material_alternate_walls') is not the same as path.backwards, most notably in the sense that most of 'path.backwards' is taken care of inside of the appropriate methods, but the backwards specifically for this mode is only done in one or two very specific spots _outside of methods such as addWalls_. While most other things where already taken care of, the start-vertex wasn't yet.

CURA-9294

Ultimaker/CuraEngine,Ultimaker/CuraEngine