heif: item location base offset and data reference index are ignored

[Peter Grant]

When parsing heif files, the BaseOffset in the item location box isn't parsed and isn't referenced anywhere. This was hidden by the decoding error described in #47. Similarly, DataReferenceIndex is not used anywhere; non-zero values in this field imply that the extent offsets are relative to something other than the start of the file. These discrepancies impact only the EXIF() function or calling code that reads data from box locations.