Blender Git Loki

Fix broken atomic_cas lock in own recent commit in bmesh.

Using atomic cas correctly is really hairy... ;)

In this case, the returned value from cas needs to validate *two*
conditions, it must not be FLT_MAX (which is our 'locked' value and
would mean another thread has already locked it), but it also must be
equal to previously stored value...

This means we need two steps per loop here, hence using a 'for' loop
instead of a 'while' one now.

Note that collisions are (as expected) very rare, less than 1 for 10k
typically, so did not catch the issue initially (also because I was
mostly working with release build to check on performances...).