Raytracing example doesnt compile with threads#177

nim version: `1.4.8 [Linux: amd64]` Error: ```shell raytracing ) nim c -d:danger --threads:on smallpt.nim Hint: used config file '/etc/nim/nim.cfg' [Conf] Hint: used config file '/etc/nim/config.nims' [Conf] Hint: used config file '/home/gavr/Projects/nim/weave-master/nim.cfg' [Conf] .................................... /home/gavr/Projects/nim/weave-master/weave/cross_thread_com/scoped_barriers.nim(66, 12) Warning: Moving a shared resource (an atomic type). [User] ..................................Traceback from system (most recent call last) Error: unhandled exception: 'sym' is not accessible using discriminant 'kind' of type 'TNode' [FieldDefect] ``` With threads on all works: ```shell raytracing ) nim c -d:danger --threads:off smallpt.nim Hint: used config file '/etc/nim/nim.cfg' [Conf] Hint: used config file '/etc/nim/config.nims' [Conf] Hint: used config file '/home/gavr/Projects/nim/weave-master/nim.cfg' [Conf] ...................CC: stdlib_formatfloat.nim CC: stdlib_io.nim CC: stdlib_system.nim CC: stdlib_parseutils.nim CC: stdlib_unicode.nim CC: stdlib_strutils.nim CC: stdlib_strformat.nim CC: stdlib_pathnorm.nim CC: stdlib_times.nim CC: stdlib_os.nim CC: smallpt.nim Hint: [Link] Hint: 52978 lines; 1.761s; 61.355MiB peakmem; Dangerous Release build; proj: /home/gavr/Projects/nim/weave-master/demos/raytracing/smallpt.nim; out: /home/gavr/Projects/nim/weave-master/demos/raytracing/smallpt [SuccessX] ```