Changed polygonizer, zoom behaviour

This release fixes a bug in the PolargraphSD firmware, and adds a couple of features to
the Polargraph Controller.

polargraphcontroller 2.4.0

• Added: Feature to swap the POLYGONIZER style that Geomerative uses when dealing
  with shapes. Currently only two are supported - UNIFORMLENGTH (1), which also uses a
  numberspinner to specify the actual uniform length, and ADAPTATIVE. Prior to v2.2.2,
  polargraphcontroller used ADAPTATIVE, but http://www.polargraph.co.uk/forum/polargraphs-group2/troubleshooting-forum5/saw-tooth-circle-and-dubious-text-thread395/
  prompted me to change it to UNIFORMLENGTH. But then http://www.polargraph.co.uk/forum/polargraphs-group2/troubleshooting-forum5/svg-differences-between-polargraphcontroller-2-1-1-2-3-0-thread523.0/#postid-3106
  has prompted this more recent change. Horses for courses, right?
• Fixed: Zooming would always keep the machine origin fixed, so you'd have to
  periodically grab and re-centre the preview machine. Now the position of the
  mouse on the machine is the origin, and the machine zooms around it. Should have
  always been like that.
• Fixed: Zooming speed did not scale. Now one "spin" on the wheel zooms
  the same sized increment no matter the current zoom level.
• Found: Changing the window size by stretching the edges does properly
  reflow the buttons and enable them. Maximising the window does not. So
  manually roll the window out to the size you want, rather than maximising it
  (Windows 10).

polargraph_firmware_polarshield 1.3.1

• Fixed: The pen lift height command was borked. Now unborked. Or at least,
  borked in a more functional way.