macOS target

[mems]

How to compile on macOS:

1. rename png.h and png.cpp to something like png_io.* update includes path in alpha-bleeding.cpp alpha-remove.cpp alpha-set.cpp → #include "png_io.h"
2. add #include "png_io.h" to png_io.cpp
3. update in makefile src/png.h→src/png_io.h
4. add -I/opt/local/include -L/opt/local/lib as g++ option in makefile
5. in makefile remove .exe