+ /*
+ * For each face, we optionally compute and store its 'incentre'.
+ * The incentre of a triangle is the centre of a circle tangent to
+ * all three edges; I generalise the concept to arbitrary polygons
+ * by defining it to be the centre of the largest circle you can fit
+ * anywhere in the polygon. It's a useful thing to know because if
+ * you want to draw any symbol or text in the face (e.g. clue
+ * numbers in Loopy), that's the place it will most easily fit.
+ *
+ * When a grid is first generated, no face has this information
+ * computed, because it's fiddly to do. You can call
+ * grid_find_incentre() on a face, and it will fill in ix,iy below
+ * and set has_incentre to indicate that it's done so.
+ */
+ int has_incentre;
+ int ix, iy; /* incentre (centre of largest inscribed circle) */