LibGfx/WebP: Extract a decode_webp_image_data() function

That way, animated and non-animated webp files use the same code path
to decode images. That will make it easier to add handling for lossy
decompression and for alpha chunk handling.

No behavior change.

Userland/Libraries/LibGfx/ImageFormats/WebPLoader.cpp

@@ -1576,6 +1576,19 @@ static ErrorOr<ImageData> decode_webp_animation_frame_image_data(WebPLoadingCont
     return image_data;
 }
 
+static ErrorOr<NonnullRefPtr<Bitmap>> decode_webp_image_data(WebPLoadingContext& context, ImageData const& image_data)
+{
+    if (image_data.image_data_chunk->type == FourCC("VP8L")) {
+        VERIFY(!image_data.alpha_chunk.has_value());
+        return decode_webp_chunk_VP8L(context, image_data.image_data_chunk.value());
+    }
+
+    VERIFY(image_data.image_data_chunk->type == FourCC("VP8 "));
+
+    // FIXME: Implement.
+    return context.error("WebPImageDecoderPlugin: decoding lossy webps not yet implemented");
+}
+
 // https://developers.google.com/speed/webp/docs/riff_container#assembling_the_canvas_from_frames
 static ErrorOr<ImageFrameDescriptor> decode_webp_animation_frame(WebPLoadingContext& context, size_t frame_index)
 {
@@ -1615,10 +1628,7 @@ static ErrorOr<ImageFrameDescriptor> decode_webp_animation_frame(WebPLoadingCont
         auto frame_image_data = TRY(decode_webp_animation_frame_image_data(context, frame_description));
         VERIFY(frame_image_data.image_data_chunk.has_value());
 
-        if (frame_image_data.image_data_chunk->type == FourCC("VP8 "))
-            return context.error("WebPImageDecoderPlugin: decoding lossy webps not yet implemented");
-
-        auto frame_bitmap = TRY(decode_webp_chunk_VP8L(context, frame_image_data.image_data_chunk.value()));
+        auto frame_bitmap = TRY(decode_webp_image_data(context, frame_image_data));
         if (static_cast<u32>(frame_bitmap->width()) != frame_description.frame_width || static_cast<u32>(frame_bitmap->height()) != frame_description.frame_height)
             return context.error("WebPImageDecoderPlugin: decoded frame bitmap size doesn't match frame description size");
 
@@ -1741,33 +1751,29 @@ ErrorOr<ImageFrameDescriptor> WebPImageDecoderPlugin::frame(size_t index)
         TRY(decode_webp_chunks(*m_context));
 
     if (is_animated()) {
-        if (m_context->state < WebPLoadingContext::State::AnimationFrameChunksDecoded) {
+        if (m_context->state < WebPLoadingContext::State::AnimationFrameChunksDecoded)
             TRY(decode_webp_animation_frame_chunks(*m_context));
-        }
         return decode_webp_animation_frame(*m_context, index);
     }
 
-    if (m_context->image_data.image_data_chunk.has_value() && m_context->image_data.image_data_chunk->type == FourCC("VP8L")) {
-        if (m_context->state < WebPLoadingContext::State::BitmapDecoded) {
-            auto bitmap = TRY(decode_webp_chunk_VP8L(*m_context, m_context->image_data.image_data_chunk.value()));
+    if (!m_context->image_data.image_data_chunk.has_value())
+        return m_context->error("WebPImageDecoderPlugin: Did not find image data chunk");
 
-            // Check that size in VP8X chunk matches dimensions in VP8L chunk if both are present.
-            if (m_context->first_chunk->type == FourCC("VP8X")) {
-                if (static_cast<u32>(bitmap->width()) != m_context->vp8x_header.width)
-                    return m_context->error("WebPImageDecoderPlugin: VP8X and VP8L chunks store different widths");
-                if (static_cast<u32>(bitmap->height()) != m_context->vp8x_header.height)
-                    return m_context->error("WebPImageDecoderPlugin: VP8X and VP8L chunks store different heights");
-            }
+    if (m_context->state < WebPLoadingContext::State::BitmapDecoded) {
+        auto bitmap = TRY(decode_webp_image_data(*m_context, m_context->image_data));
 
-            m_context->bitmap = move(bitmap);
-            m_context->state = WebPLoadingContext::State::BitmapDecoded;
+        // Check that size in VP8X chunk matches dimensions in VP8 or VP8L chunk if both are present.
+        if (m_context->first_chunk->type == FourCC("VP8X")) {
+            if (static_cast<u32>(bitmap->width()) != m_context->vp8x_header.width || static_cast<u32>(bitmap->height()) != m_context->vp8x_header.height)
+                return m_context->error("WebPImageDecoderPlugin: VP8X and VP8/VP8L chunks store different dimensions");
         }
 
-        VERIFY(m_context->bitmap);
-        return ImageFrameDescriptor { m_context->bitmap, 0 };
+        m_context->bitmap = move(bitmap);
+        m_context->state = WebPLoadingContext::State::BitmapDecoded;
     }
 
-    return Error::from_string_literal("WebPImageDecoderPlugin: decoding lossy webps not yet implemented");
+    VERIFY(m_context->bitmap);
+    return ImageFrameDescriptor { m_context->bitmap, 0 };
 }
 
 ErrorOr<Optional<ReadonlyBytes>> WebPImageDecoderPlugin::icc_data()