[AVIF Downlevels] Allow pixi property to be missing in AVIF images

https://bugs.webkit.org/show_bug.cgi?id=270180
rdar://116808395

Reviewed by Cameron McCormack.

If an image is missing the pixi property, decoding its frames will fail because
we enable AVIF_STRICT_PIXI_REQUIRED when avifDecoder is created. So we need to
remove this restriction to be compatible with newer OSs and other browsers.

* Source/WebCore/platform/image-decoders/avif/AVIFImageReader.cpp:
(WebCore::AVIFImageReader::AVIFImageReader):

Canonical link: https://commits.webkit.org/275608@main

Source/WebCore/platform/image-decoders/avif/AVIFImageReader.cpp

@@ -37,6 +37,11 @@ AVIFImageReader::AVIFImageReader(RefPtr<AVIFImageDecoder>&& decoder)
     : m_decoder(WTFMove(decoder))
     , m_avifDecoder(avifDecoderCreate())
 {
+    // Allow the PixelInformationProperty ('pixi') to be missing in AV1 image
+    // items. libheif v1.11.0 or older does not add the 'pixi' item property to
+    // AV1 image items. (This issue has been corrected in libheif v1.12.0.).
+    // See the definition of AVIF_STRICT_PIXI_REQUIRED in avif.h.
+    m_avifDecoder->strictFlags &= ~AVIF_STRICT_PIXI_REQUIRED;
 }
 
 AVIFImageReader::~AVIFImageReader() = default;