LibGfx: Add DeltaE() function

Userland/Libraries/LibGfx/DeltaE.h

@@ -0,0 +1,21 @@
+/*
+ * Copyright (c) 2023, Nico Weber <thakis@chromium.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <LibGfx/CIELAB.h>
+
+namespace Gfx {
+
+// Returns a number between 0 and 100 that describes how far apart two colors are in human perception.
+// A return value < 1 means that the two colors are not noticeably different.
+// The larger the return value, the easier it is to tell the two colors apart.
+// Works better for colors that are somewhat "close".
+//
+// You can use ICC::sRGB()->to_lab() to convert sRGB colors to CIELAB.
+float DeltaE(CIELAB const&, CIELAB const&);
+
+}