fix(png): Write out proper tiff header version in png EXIF blobs (#3984)

When writing out EXIF headers in PNG files, the `tiff_version` field
wasn't handling endianness properly.

This lead to some tools like `exiv2` refusing to process the file at
all[1]

[1] Exiv2/exiv2#2745

---------

Signed-off-by: Jesse Yurkovich <jesse.y@gmail.com>

src/libOpenImageIO/exif.cpp

@@ -1302,6 +1302,8 @@ encode_exif(const ImageSpec& spec, std::vector<char>& blob,
     TIFFHeader head;
     head.tiff_magic   = (endianreq == endian::little) ? 0x4949 : 0x4d4d;
     head.tiff_version = 42;
+    if (endianreq != endian::native)
+        swap_endian(&head.tiff_version);
     // N.B. need to swap_endian head.tiff_diroff  below, once we know the sizes
     append(blob, head);