LibGfx+LibWeb: Add ImmutableBitmap for images bitmap caching in painter

Before this change, we used Gfx::Bitmap to represent both decoded
images that are not going to be mutated and bitmaps corresponding
to canvases that could be mutated.

This change introduces a wrapper for bitmaps that are not going to be
mutated, so the painter could do caching: texture caching in the case
of GPU painter and potentially scaled bitmap caching in the case of CPU
painter.

Userland/Libraries/LibGfx/ImmutableBitmap.cpp

@@ -0,0 +1,24 @@
+/*
+ * Copyright (c) 2023, Aliaksandr Kalenik <kalenik.aliaksandr@gmail.com>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include <LibGfx/ImmutableBitmap.h>
+
+namespace Gfx {
+
+static size_t s_next_immutable_bitmap_id = 0;
+
+NonnullRefPtr<ImmutableBitmap> ImmutableBitmap::create(NonnullRefPtr<Bitmap> bitmap)
+{
+    return adopt_ref(*new ImmutableBitmap(move(bitmap)));
+}
+
+ImmutableBitmap::ImmutableBitmap(NonnullRefPtr<Bitmap> bitmap)
+    : m_bitmap(move(bitmap))
+    , m_id(s_next_immutable_bitmap_id++)
+{
+}
+
+}