em/gcanvas — pull request #9 by tmpvar (2014-05-20 14:56:35+00:00)
Title: allow for inverted Z axis

I've been using grbl with a bunch of gcode generators which all seem to assume that -Z is towards the work and +Z is where you retract to. This was mind boggling for me at first, and you can see my concerns in grbl/grbl#230

These changes should be backwards compatible with the previous behavior, but allows me to use gcanvas (nice work btw!)