gcode2ps updates - now with isometric view

[Henner Zeller]

Hi,
so as a preparation (procrastination?) to work on the planner over the
weekend I updated gcode2ps to also allow different views than just
x/y; any rotation is possible, but probably the most useful is
-Visometric to show a common oblique view of what is happening in 3D space.

https://github.com/hzeller/beagleg/blob/master/Development.md#gcode2ps

I made the PostScript interpreter calculate the 3D -> 2D projection,
so that was interesting and allows for some superfluous feature:
creating animations :)
(also posted here https://plus.google.com/u/0/+HennerZeller/posts/aQ1kMZnL8V6 )

The 'make test-html' thing in src/ now generates the isometric view by
default, but the tool should probably have an auto-mode that only
shows isometric view if there is happening much in three dimensions,
otherwise choose the two-dimensional plane where things are happening.

Happy CNC-ing,
-h