Allow JPEGtoRAW to open PNG source files

Hello.

Since all modern web-sites and cameras/smartphones save JPEGs at rather high quality there is little use for JPEGtoRAW for current images. One area where images do still suffer from compression artifacts is PDF files, though.

Unfortunately my favorite PDF software exports images as PNG files (makes sense to keep them lossless). So in order for me to send these through JPEGtoRAW I first have to convert them to JPEG, which is an extra step and may lead to additional compression artifacts.

Allowing JPEGtoRAW to open PNG files would make things a lot easier for me.