document pt_image_tile_* as semi-threadsafe, and nogil pypngtile.pyx (and get rid of trap_err, ugh). Also fix constness warning
#ifndef SHARED_UTIL_H
#define SHARED_UTIL_H
#include <stddef.h>
/**
* Replace the file extension in the given path buffer with the given extension, which should be of the form ".foo"
*/
int chfext (char *path, size_t len, const char *newext);
/**
* Copy filesystem path with new file extension
*/
int path_with_fext (const char *path, char *buf, size_t len, const char *newext);
#endif