memoryformat: Add test for depth merging

We want to ensure a few invariants:
- merge (a, a) == a
- merge (a, b) == merge (b, a)

Add tests to verify that this is now the case.

testsuite/gdk/memoryformat.c

@@ -0,0 +1,25 @@
+#include <gdk/gdk.h>
+#include <gdk/gdkmemoryformatprivate.h>
+
+static void
+test_depth_merge (void)
+{
+  for (GdkMemoryDepth depth1 = GDK_MEMORY_U8; depth1 < GDK_N_DEPTHS; depth1++)
+    {
+      g_assert_cmpint (gdk_memory_depth_merge (depth1, depth1), ==, depth1);
+      for (GdkMemoryDepth depth2 = GDK_MEMORY_U8; depth2 < depth1; depth2++)
+        {
+          g_assert_cmpint (gdk_memory_depth_merge (depth1, depth2), ==, gdk_memory_depth_merge (depth2, depth1));
+        }
+    }
+}
+
+int
+main (int argc, char *argv[])
+{
+  (g_test_init) (&argc, &argv, NULL);
+
+  g_test_add_func ("/depth/merge", test_depth_merge);
+
+  return g_test_run ();
+}

testsuite/gdk/meson.build

@@ -63,6 +63,7 @@ internal_tests = [
   { 'name': 'texture' },
   { 'name': 'gltexture' },
   { 'name': 'subsurface' },
+  { 'name': 'memoryformat' },
 ]
 
 if os_linux