disable check for cyclic iref references (#993)

libheif/box.cc

@@ -2652,6 +2652,20 @@ Error Box_iref::parse(BitstreamRange& range)
   }
 
 
+#if 0
+  // Note: This input sanity check does not work as expected.
+  // Its idea was to prevent infinite recursions while decoding when the input file
+  // contains cyclic references. However, apparently there are cases where cyclic
+  // references are actually allowed, like with images that have premultiplied alpha channels:
+  // | Box: iref -----
+  // | size: 40   (header size: 12)
+  // | reference with type 'auxl' from ID: 2 to IDs: 1
+  // | reference with type 'prem' from ID: 1 to IDs: 2
+  //
+  // TODO: implement the infinite recursion detection in a different way. E.g. by passing down
+  //       the already processed item-ids while decoding an image and checking whether the current
+  //       item has already been decoded before.
+
   // --- check for cyclic references
 
   for (const auto& ref : m_references) {
@@ -2687,6 +2701,7 @@ Error Box_iref::parse(BitstreamRange& range)
       }
     }
   }
+#endif
 
   return range.get_error();
 }