LibGfx: Add a simple GrayscaleBitmap class

This is very similar to the existing CharacterBitmap class but instead
intended for small static grayscale bitmaps (such as signed distance
fields).

Userland/Libraries/LibGfx/GrayscaleBitmap.h

@@ -0,0 +1,38 @@
+/*
+ * Copyright (c) 2023, MacDue <macdue@dueutil.tech>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include "Size.h"
+#include <AK/RefCounted.h>
+#include <AK/RefPtr.h>
+#include <AK/StringView.h>
+
+namespace Gfx {
+
+class GrayscaleBitmap {
+public:
+    GrayscaleBitmap() = delete;
+    constexpr GrayscaleBitmap(ReadonlyBytes data, unsigned width, unsigned height)
+        : m_data(data)
+        , m_size(width, height)
+    {
+        VERIFY(width * height == data.size());
+    }
+
+    constexpr u8 pixel_at(unsigned x, unsigned y) const { return m_data[y * width() + x]; }
+    constexpr ReadonlyBytes data() const { return m_data; }
+
+    constexpr IntSize size() const { return m_size; }
+    constexpr unsigned width() const { return m_size.width(); }
+    constexpr unsigned height() const { return m_size.height(); }
+
+private:
+    ReadonlyBytes m_data {};
+    IntSize m_size {};
+};
+
+}