LibGfx: Parse some of WebP 'VP8 ' chunk

This is for lossy compression, in which case a WebP file is
a single VP8 key frame.

This only parses the 10-byte frame header, which contains image
dimensions (and some other things).

For now, just dbgln_if() all data. Eventually we'll want to use at
least width and height.

Userland/Libraries/LibGfx/WebPLoader.cpp

@@ -147,11 +147,58 @@ static ErrorOr<Chunk> decode_webp_advance_chunk(WebPLoadingContext& context, Rea
 }
 
 // https://developers.google.com/speed/webp/docs/riff_container#simple_file_format_lossy
+// https://datatracker.ietf.org/doc/html/rfc6386#section-19 "Annex A: Bitstream Syntax"
 static ErrorOr<void> decode_webp_simple_lossy(WebPLoadingContext& context, Chunk const& vp8_chunk)
 {
-    // FIXME
-    (void)context;
-    (void)vp8_chunk;
+    VERIFY(vp8_chunk.type == FourCC("VP8 "));
+
+    if (vp8_chunk.data.size() < 10)
+        return context.error("WebPImageDecoderPlugin: 'VP8 ' chunk too small");
+
+    // FIXME: Eventually, this should probably call into LibVideo/VP8,
+    // and image decoders should move into LibImageDecoders which depends on both LibGfx and LibVideo.
+    // (LibVideo depends on LibGfx, so LibGfx can't depend on LibVideo itself.)
+
+    // https://datatracker.ietf.org/doc/html/rfc6386#section-4 "Overview of Compressed Data Format"
+    // "The decoder is simply presented with a sequence of compressed frames [...]
+    //  The first frame presented to the decompressor is [...] a key frame.  [...]
+    //  [E]very compressed frame has three or more pieces. It begins with an uncompressed data chunk comprising 10 bytes in the case of key frames
+
+    u8 const* data = vp8_chunk.data.data();
+
+    // https://datatracker.ietf.org/doc/html/rfc6386#section-9.1 "Uncompressed Data Chunk"
+    u32 frame_tag = data[0] | (data[1] << 8) | (data[2] << 16);
+    bool is_key_frame = (frame_tag & 1) == 0; // https://www.rfc-editor.org/errata/eid5534
+    u8 version = (frame_tag & 0xe) >> 1;
+    bool show_frame = (frame_tag & 0x10) != 0;
+    u32 size_of_first_partition = frame_tag >> 5;
+
+    if (!is_key_frame)
+        return context.error("WebPImageDecoderPlugin: 'VP8 ' chunk not a key frame");
+
+    // FIXME: !show_frame does not make sense in a webp file either, probably?
+
+    u32 start_code = data[3] | (data[4] << 8) | (data[5] << 16);
+    if (start_code != 0x2a019d) // https://www.rfc-editor.org/errata/eid7370
+        return context.error("WebPImageDecoderPlugin: 'VP8 ' chunk invalid start_code");
+
+    // "The scaling specifications for each dimension are encoded as follows.
+    //   0     | No upscaling (the most common case).
+    //   1     | Upscale by 5/4.
+    //   2     | Upscale by 5/3.
+    //   3     | Upscale by 2."
+    // This is a display-time operation and doesn't affect decoding.
+    u16 width_and_horizontal_scale = data[6] | (data[7] << 8);
+    u16 width = width_and_horizontal_scale & 0x3fff;
+    u8 horizontal_scale = width_and_horizontal_scale >> 14;
+
+    u16 heigth_and_vertical_scale = data[8] | (data[9] << 8);
+    u16 height = heigth_and_vertical_scale & 0x3fff;
+    u8 vertical_scale = heigth_and_vertical_scale >> 14;
+
+    dbgln_if(WEBP_DEBUG, "version {}, show_frame {}, size_of_first_partition {}, width {}, horizontal_scale {}, height {}, vertical_scale {}",
+        version, show_frame, size_of_first_partition, width, horizontal_scale, height, vertical_scale);
+
     return {};
 }