Garbage after XMP in PNG files when XMP has shrunk

1. Add a large XMP block to a PNG file (using your tool of choice).
2. Replace it with a shorter XMP block using XMP Toolkit SDK.
3. Inspect the `iTXt` segment, e.g. with this Python-script:
```
from sys import argv, stdout
from PIL import Image

stdout.reconfigure(encoding='utf-8')

with Image.open(argv[1]) as img:
img.load()
text_data = getattr(img, "text", {})

if text_data:
for key, value in text_data.items():
if key == 'XML:com.adobe.xmp':
print(value)
```

Result: The `iTXt` segment size does not change, and there are garbage XMP fragments after the valid XMP.

This seems to be because `PNG_MetaHandler::UpdateFile()` only replaces the number of bytes taken up by the new XMP block in the `iTXt` segment and not the entire segment.

I will submit a PR with a proposed fix.

Kind Regards
Bjørn Thirud

Start with PNG_MetaHandler::UpdateFile() and reproduce the reported steps using the provided Python script to inspect the iTXt segment. Compare the segment after replacing large XMP with shorter XMP; done means the segment reflects the shorter block without trailing garbage XMP fragments.

Written by the indexing model from the issue text.