Blender Git Loki

OSX vs OpenMP : implement workaround to fix crashes when using mop from a background thread

Fix# 20043 & 20392

The issue is that OSX lib does not implement TLS (Thread Local Storage), so libgomp uses pthread functions to read/write thread specific vars.
But this implementation is currently (gcc 4.2) buggy : the write function is called only at lib start (in main thread), and the var is undefined for background thread.

The workaround is to perform this gomp_tls_key var write at beginning of background threads that use openMP. (Currently: render & fluidsim)