Don't take the head of an empty list
Fix sign error in direction of straight segments
code cleanup, no change of meaning
Copy directions from control joins also

CopyDirsR is only called after controlPtDirs, so the correct direction
will be copied.
Add more Haddock docs for Metafont [ci skip]
Merge pull request #24 from diagrams/metafont-bugfix

Metafont bugfix