Consolidate duplicate implementations of compare_integers

Both Inertia and Twiddle previously included static implementations of this
exact same function, which was passed to `qsort()` as a comparator. With
this change, a single global implementation is provided in misc.c, which
will hopefully reduce code duplication going forward.

I'm refactoring this in preparation for the upcoming fallback polygon fill
function, which I'm about to add.

misc.c

@@ -351,6 +351,17 @@ void draw_rect_corners(drawing *dr, int cx, int cy, int r, int col)
     draw_line(dr, cx + r, cy + r, cx + r/2, cy + r, col);
 }
 
+int compare_integers(const void *av, const void *bv) {
+    const int *a = (const int *)av;
+    const int *b = (const int *)bv;
+    if (*a < *b)
+	return -1;
+    else if (*a > *b)
+	return +1;
+    else
+	return 0;
+}
+
 char *move_cursor(int button, int *x, int *y, int maxw, int maxh, bool wrap,
     bool *visible)
 {