Support 16Bit png files!

[AmmarkoV]

Hello,

This is a feature request to support 16bit png files.
For some use cases (like depth maps)
e.g. http://ammar.gr/depth16.png
256 values per pixel are not enough and so 16bit encoded png files are needed.

When trying to use

wget http://ammar.gr/depth16.png
file depth16.png 
 ./zpng -c depth16.png depth16.zpng
 ./zpng -d depth16.zpng depthRecoded.png
file depthRecoded.png 

output is

depth16.png: PNG image data, 1500 x 844, 16-bit grayscale, non-interlaced
depthRecoded.png: PNG image data, 1500 x 844, 8-bit grayscale, non-interlaced