renamed is_transparent (more like other function names), and exposed it to other C files

src/overviewer.h

@@ -36,6 +36,7 @@ PyObject *alpha_over_wrap(PyObject *self, PyObject *args);
 PyObject *brightness(PyObject *img, double factor);
 
 /* in iterate.c */
+int is_transparent(PyObject* tup, unsigned char b);
 PyObject *chunk_render(PyObject *self, PyObject *args);
 int init_chunk_render(void);