Blender Git Loki

Cleanup: BKE_appdir left paths set even when not found

Internally appdir functions would test if a path exists,
returning false if it doesn't, leaving the string set instead
of clearing it.

This is error prone as invalid paths could be used accidentally.

Since BKE_appdir_folder_id_user_notest & BKE_appdir_folder_id_version
depend on this, add a 'check_is_dir' argument so the path can be used
even when the directory can't be found.