spectre_adjacent: optionally report dst_edge.

Previously, you'd ask this function 'What lies on the other side of
edge #i of this Spectre tile?' and it would tell you the identity of
another Spectre. Now it will also tell you which _edge_ of that
Spectre adjoins the specified edge of the input one. This will be used
in the extra spectre-test mode I'm about to add.

spectre-internal.h

@@ -243,7 +243,7 @@ void spectrectx_step_hex(SpectreContext *ctx, SpectreCoords *sc,
  * delivering both plane and combinatorial coordinates as they go */
 Spectre *spectre_initial(SpectreContext *ctx);
 Spectre *spectre_adjacent(SpectreContext *ctx, const Spectre *src_spec,
-                          unsigned src_edge);
+                          unsigned src_edge, unsigned *dst_edge);
 
 /* For extracting the point coordinates */
 typedef struct Coord {