WebP/Lossless: Set alpha to 0xff if is_alpha_used is false in header

simple-vp8l-alpha-used-false.webp is a copy of simple-vp8l.webp,
with the byte at offset 0x18 changed from 0x10 to 0x00 -- that
is, the bit in the VP8L header that stores `is_alpha_used` is cleared.

We would already allocated a BGRx8888 instead of a BGRA8888 bitmap,
but keep actual alpha data in the `x` channel.

That lead to at least `image` still writing a PNG with an alpha channel.
So explicitly set the alpha channel to 0xff when is_alpha_used is false,
to make sure all consumers of decoded lossless webp data have behavior
consistent with other webp readers.

In practice, webp encoders usually don't write files that have
`is_alpha_used` set to false and then write actual alpha data to their
output. So this is rarely observable. However, for example for
lossy+ALPH webp files, the lossless webp used to store the ALPH channel
has `is_alpha_used` set to false and all channels but green are 0
(since the lossless green channel stores the alpha channel of a
lossy+ALPH webp). So if we dump such a bitmap to a standalone webp
file (e.g. with the temporary debugging code in fc3249a1ca8f42e72),
then without this commit here, `image` would convert that webp to
a fully transparent webp, while other webp software would correctly
display the green image with opaque alpha.

Tests/LibGfx/TestImageDecoder.cpp

@@ -342,10 +342,33 @@ TEST_CASE(test_webp_simple_lossless)
     auto frame = MUST(plugin_decoder->frame(0));
     EXPECT_EQ(frame.image->size(), Gfx::IntSize(386, 395));
 
+    EXPECT_EQ(frame.image->get_pixel(0, 0), Gfx::Color(0, 0, 0, 0));
+
     // This pixel tests all predictor modes except 5, 7, 8, 9, and 13.
     EXPECT_EQ(frame.image->get_pixel(289, 332), Gfx::Color(0xf2, 0xee, 0xd3, 255));
 }
 
+TEST_CASE(test_webp_simple_lossless_alpha_used_false)
+{
+    // This file is identical to simple-vp8l.webp, but the `is_alpha_used` used bit is false.
+    // The file still contains alpha data. This tests that the decoder replaces the stored alpha data with 0xff if `is_alpha_used` is false.
+    auto file = MUST(Core::MappedFile::map(TEST_INPUT("simple-vp8l-alpha-used-false.webp"sv)));
+    EXPECT(Gfx::WebPImageDecoderPlugin::sniff(file->bytes()));
+    auto plugin_decoder = MUST(Gfx::WebPImageDecoderPlugin::create(file->bytes()));
+    MUST(plugin_decoder->initialize());
+
+    EXPECT_EQ(plugin_decoder->frame_count(), 1u);
+    EXPECT(!plugin_decoder->is_animated());
+    EXPECT(!plugin_decoder->loop_count());
+
+    EXPECT_EQ(plugin_decoder->size(), Gfx::IntSize(386, 395));
+
+    auto frame = MUST(plugin_decoder->frame(0));
+    EXPECT_EQ(frame.image->size(), Gfx::IntSize(386, 395));
+
+    EXPECT_EQ(frame.image->get_pixel(0, 0), Gfx::Color(0, 0, 0, 0xff));
+}
+
 TEST_CASE(test_webp_extended_lossy)
 {
     // This extended lossy image has an ALPH chunk for (losslessly compressed) alpha data.

Userland/Libraries/LibGfx/ImageFormats/WebPLoaderLossless.cpp

@@ -1000,6 +1000,9 @@ ErrorOr<NonnullRefPtr<Bitmap>> decode_webp_chunk_VP8L_contents(VP8LHeader const&
     for (auto const& transform : transforms.in_reverse())
         bitmap = TRY(transform->transform(bitmap));
 
+    if (!vp8l_header.is_alpha_used)
+        bitmap->strip_alpha_channel();
+
     return bitmap;
 }