path: add function to split URL into prefix and path

Used in the following commit.

options/path.h

@@ -75,6 +75,8 @@ bool mp_path_isdir(const char *path);
 
 bool mp_is_url(bstr path);
 
+bstr mp_split_proto(bstr path, bstr *out_url);
+
 void mp_mk_config_dir(struct mpv_global *global, char *subdir);
 
 #endif /* MPLAYER_PATH_H */