Adapt C++ code for PyPy's Numpy C-API

- src/_png.cpp, src/_path.cpp, src/_image.cpp: Replace direct field
  references of PyArrayObject's by API functions.
- src/_path.cpp: Simplify snprintf()'s in svg generator
- src/_image.cpp, src/image.h: Remove fromarray2() method, since it was
  clearly broken (unsigned char *data; x = *(double*)(data++);) and not
  referenced from anywhere.