[hentaifox]: 404 errors due to extractor not supporting PNG

[jgrubs1]

Some galleries on hentaifox are available in high resolution, with PNG instead of JPG images. The extractor does not seem to recognize this, thus missing the high-quality images. This leads to download errors. Example:

gallery-dl -v https://hentaifox.com/gallery/80550

This results in 404 errors on images 2, 12, 13, 26, 28, 45, 46, and 48:

[urllib3.connectionpool][debug] https://i.hentaifox.com:443 "GET /003/1859568/2.jpg HTTP/1.1" 404 None
[downloader.http][warning] '404 Not Found' for 'https://i.hentaifox.com/003/1859568/2.jpg'
[download][error] Failed to download hentaifox_80550_002.jpg

The correct URLs, as parsed from the gallery HTML, are as follows:

https://i.hentaifox.com/003/1859568/2.png
through
https://i2.hentaifox.com/003/1859568/48.png

Suggested fix: patch hentaifox extractor to detect the availability of PNG files in the gallery, and automatically download full-size PNG images instead of the downsized JPG, when present.