New Loopy tiling: 'Great Great Dodecagonal'.

Officially known as the '3-4-6-12 tiling', according to, e.g.,
https://en.wikipedia.org/wiki/3-4-6-12_tiling .

Thanks to Michael Quevillon for contributing this patch (and for
choosing a less hard-to-remember name for the tiling :-).

grid.h

@@ -105,6 +105,7 @@ typedef struct grid {
   A(FLORET,floret) \
   A(DODECAGONAL,dodecagonal) \
   A(GREATDODECAGONAL,greatdodecagonal) \
+  A(GREATGREATDODECAGONAL,greatgreatdodecagonal) \
   A(PENROSE_P2,penrose_p2_kite) \
   A(PENROSE_P3,penrose_p3_thick)