Add Sandbox Libpng

[alexelex]

Adding LibPNG Sandbox
sapi version LibPNG using wrapper for functions with callbacks and internal libpng errors
sandboxed version contains some functions for using std::FILE.
2 Google Tests, for testing run ./tests/tests from build
2 example, run with

./examples/pngtopng /absolute/path/to/input /absolute/path/to/output

./examples/rgbtobgr /absolute/path/to/input /absolute/path/to/output

2 images and 2 result images for these examples, pngtopng used pngtest.png, rgbtobgr used red_ball.png