Resolves #3796

#3041 originally asked why PNGs were opened in I mode rather than I;16, and the response was

There's better support for image operations on int32 images than int16, especially for signed 16 bit images.

Nevertheless, #3796 was opened, and has received various comments.

The one I find compelling is #3041 (comment)

Pillow's TIFF and JPEG handlers, for example, will happily open 16-bit data without upcasting, so I don't see why the PNG handler can't do the same.

Looking through our test images, it is true that 12bit.cropped.tif is opened as I;16, 16bit.MM.cropped.tif is I;16B and 16bit.cropped.j2k is I;16. So some consistency would be good.

This PR changes PNG to open as I;16 rather than I.