gentoo ebuild added + new libpng fixed

Hello. You are using png_sizeof macro, which does not exists in libpng >= 15. Please see
http://sourceforge.net/p/libpng/code/ci/libpng12/tree/pngconf.h
```
/* This is usually size_t.  It is typedef'ed just in case you need it to
change (I'm not sure if you will or not, so I thought I'd be safe) */
#ifdef PNG_SIZE_T
   typedef PNG_SIZE_T png_size_t;
#  define png_sizeof(x) png_convert_size(sizeof(x))
#else
   typedef size_t png_size_t;
#  define png_sizeof(x) sizeof(x)
#endif
```
and
http://sourceforge.net/p/libpng/code/ci/libpng14/tree/pngconf.h
```
#define png_sizeof(x) (sizeof (x))
```
As I understand, we should use usual sizeof in libpng >= 15