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Improve connectedness-error highlighting in Singles.

Using exactly the same policy as I did for Range the other day: if
multiple regions exist, then one is taken to be canonical and all the
others are marked as errors.

[originally from svn r10233]

--- a/singles.c

+++ b/singles.c

@@ -535,16 +535,28 @@

     for (y = 0; y < h; y++) /* check rows from (0,y) */

         error += check_rowcol(state, y*w, 1, w, flags);

-    /* mark (all) white regions as an error if there is more than one.

-     * may want to make this less in-your-face (by only marking

-     * the smallest region as an error, for example -- but what if we

-     * have two regions of identical size?) */

-    for (i = 0; i < state->n; i++) {

-        if (!(state->flags[i] & F_BLACK) &&

-            dsf_size(dsf, i) < nwhite) {

-            error += 1;

-            if (flags & CC_MARK_ERRORS)

-                state->flags[i] |= F_ERROR;

+    /* If there's more than one white region, pick the largest one to

+     * be the canonical one (arbitrarily tie-breaking towards lower

+     * array indices), and mark all the others as erroneous. */

+    {

+        int largest = 0, canonical = -1;

+        for (i = 0; i < state->n; i++)

+            if (!(state->flags[i] & F_BLACK)) {

+                int size = dsf_size(dsf, i);

+                if (largest < size) {

+                    largest = size;

+                    canonical = i;

+                }

+            }

+

+        if (largest < nwhite) {

+            for (i = 0; i < state->n; i++)

+                if (!(state->flags[i] & F_BLACK) &&

+                    dsf_canonify(dsf, i) != canonical) {

+                    error += 1;

+                    if (flags & CC_MARK_ERRORS)

+                        state->flags[i] |= F_ERROR;

+                }

         }

     }