LibGfx/WebP: Move lossy decoder to its own file

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

There isn't a lot of lossy decoder yet, but it'll make implementing it
more convenient.

No behavior change.

Userland/Libraries/LibGfx/ImageFormats/WebPLoaderLossy.h

@@ -0,0 +1,31 @@
+/*
+ * 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 VP8Header {
+    u8 version;
+    bool show_frame;
+    u32 size_of_first_partition;
+    u32 width;
+    u8 horizontal_scale;
+    u32 height;
+    u8 vertical_scale;
+    ReadonlyBytes lossy_data;
+};
+
+// Parses the header data in a VP8 chunk. Pass the payload of a `VP8 ` chunk, after the tag and after the tag's data size.
+ErrorOr<VP8Header> decode_webp_chunk_VP8_header(ReadonlyBytes vp8_data);
+
+ErrorOr<NonnullRefPtr<Bitmap>> decode_webp_chunk_VP8_contents(VP8Header const&, bool include_alpha_channel);
+
+}