LibGfx: Add BitmapMixer

With this BitmapMixer one can draw one Bitmap onto another with
different modes.
For now the only supported mixing methods implemented are Add and
Lightest (which is very naive).

Userland/Libraries/LibGfx/BitmapMixer.h

@@ -0,0 +1,29 @@
+/*
+ * Copyright (c) 2022, Tobias Christiansen <tobyase@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include "Bitmap.h"
+
+namespace Gfx {
+
+class BitmapMixer {
+public:
+    enum class MixingMethod {
+        Add,
+        Lightest,
+    };
+
+    BitmapMixer(Bitmap& bitmap)
+        : m_bitmap(bitmap) {};
+
+    void mix_with(Bitmap&, MixingMethod);
+
+private:
+    Bitmap& m_bitmap;
+};
+
+}