Loopy / grid.c: support the new Spectre monotiling.

This uses a tile shape very similar to the hat, but the tiling
_structure_ is totally changed so that there aren't any reflected
copies of the tile.

I'm not sure how much difference this makes to gameplay: the two
tilings are very similar for Loopy purposes. But the code was fun to
write, and I think the Spectre shape is noticeably prettier, so I'm
adding this to the collection anyway.

The test programs also generate a pile of SVG images used in the
companion article on my website.

auxiliary/spectre-help.h

@@ -0,0 +1,51 @@
+/*
+ * Header for spectre-help.c
+ */
+
+/* Dummy value indicating no specific hexagon, used in some diagrams
+ * for the accompanying article. */
+#define NO_HEX (Hex)9
+
+/*
+ * String constants for the hex names, including an extra entry
+ * mapping NO_HEX to the empty string.
+ */
+extern const char *hex_names[10];
+
+typedef struct Graphics {
+    FILE *fp;
+    bool close_file; /* if it's not stdout */
+    bool started;    /* have we written the header yet? */
+    double xoff, xscale, yoff, yscale, absscale, linewidth;
+    bool jigsaw_mode; /* draw protrusions on hex edges */
+    bool vertex_blobs; /* draw blobs marking hex vertices */
+    bool hex_arrows; /* draw arrows orienting each hex */
+    bool number_edges; /* number the edges of everything */
+    bool number_cells; /* number the things themselves */
+    bool four_colour;  /* four-colour Spectres instead of semantically */
+    bool arcs; /* draw Spectre edges as arcs */
+} Graphics;
+
+typedef struct GrCoords {
+    double x, y;
+} GrCoords;
+
+Graphics *gr_new(const char *filename, double xmin, double xmax,
+                 double ymin, double ymax, double scale);
+void gr_free(Graphics *gr);
+GrCoords gr_logcoords(Point p);
+GrCoords gr_log2phys(Graphics *gr, GrCoords c);
+GrCoords gr_physcoords(Graphics *gr, Point p);
+void gr_draw_text(Graphics *gr, GrCoords logpos, double logheight,
+                  const char *text);
+void gr_draw_path(Graphics *gr, const char *classes, const GrCoords *phys,
+                  size_t n, bool closed);
+void gr_draw_blob(Graphics *gr, const char *classes, GrCoords log,
+                  double logradius);
+void gr_draw_hex(Graphics *gr, unsigned index, Hex htype,
+                 const Point *vertices);
+void gr_draw_spectre(Graphics *gr, Hex container, unsigned index,
+                     const Point *vertices);
+void gr_draw_spectre_from_coords(Graphics *gr, SpectreCoords *sc,
+                                 const Point *vertices);
+void gr_draw_extra_edge(Graphics *gr, Point a, Point b);