Wrapped libpng usage in a pair of functions under PNGDriverCommon,
which convert between Godot Image and png data.
Switched to libpng 1.6 simplified API for ease of maintenance.
Implemented ImageLoaderPNG and ResourceSaverPNG in terms of
PNGDriverCommon functions.
Travis, switched to builtin libpng (thus builtin freetype and zlib also)
so we can build on Xenial.