region: add some region-region intersects tests

test/region.cpp

@@ -24,6 +24,30 @@ main (int, char **)
         tap.expect (!a.intersects (b), "simple 2d intersection");
     }
 
+    {
+        using p_t = cruft::point2i;
+        using e_t = cruft::extent2i;
+
+        struct {
+            cruft::region2i a;
+            cruft::region2i b;
+            bool intersection;
+            char const *message;
+        } const TESTS[] = {
+            {
+                .a = { p_t { 58, 22 }, e_t { 13, 8 } },
+                .b = { p_t { 58, 22 }, e_t {  7, 7 } },
+                .intersection = true,
+                .message = "Coaligned base",
+            },
+        };
+
+        for (auto const &[a, b, res, msg]: TESTS) {
+            tap.expect_eq (a.intersects (b), res, "intersection a-on-b: %!", msg);
+            tap.expect_eq (b.intersects (a), res, "intersection b-on-a: %!", msg);
+        }
+    }
+
     // check various floating point maximums successfully test for
     // intersection. the concern is that infinities are incorrectly handled
     // in some comparisons.