PNG and GIF file handling for pic.lua

This extends pic.lua with handling of PNG and GIF files.

A bit controversial might be that I chose to use lodepng instead of libpng. Lodepng has a simpler interface for decoding, without many of the bells and whistles of libpng. This corresponds with much simpler interfacing code.
I could have used mupdfimg.lua at least for PNG files. I did not do this for now since I thought it might be attractive to have a basic image reading facility that does not need the full MuPDF library loaded (which is quite big). Maybe in the future we could try to use pic.lua for the ImageWidget in Koreader's UI code - and be rewarded with a quicker start-up?

These changes introduce LodePNG and GIFLIB.

Windows compilation is untested, but should work...