LibGfx/WebP: Move lossless decoder to its own file

Pure code move (except of removing `static` on the two public functions
in the new header), not behavior change.

Userland/Libraries/LibGfx/CMakeLists.txt

@@ -48,6 +48,7 @@ set(SOURCES
     ImageFormats/QOIWriter.cpp
     ImageFormats/TGALoader.cpp
     ImageFormats/WebPLoader.cpp
+    ImageFormats/WebPLoaderLossless.cpp
     Painter.cpp
     Palette.cpp
     Path.cpp

Userland/Libraries/LibGfx/ImageFormats/WebPLoaderLossless.h

@@ -0,0 +1,25 @@
+/*
+ * Copyright (c) 2023, Nico Weber <thakis@chromium.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <AK/Span.h>
+#include <AK/Types.h>
+#include <LibGfx/Bitmap.h>
+
+namespace Gfx {
+
+struct VP8LHeader {
+    u16 width;
+    u16 height;
+    bool is_alpha_used;
+    ReadonlyBytes lossless_data;
+};
+
+ErrorOr<VP8LHeader> decode_webp_chunk_VP8L_header(ReadonlyBytes vp8l_data);
+ErrorOr<NonnullRefPtr<Bitmap>> decode_webp_chunk_VP8L_contents(VP8LHeader const& vp8l_header);
+
+}