Blender Git Loki

Fix T52816: regression can't open file in 2.79 (crash).

Tentative fix, since I cannot reproduce thenissue for some reason here
on linux.

Core of the problem is pretty clear though, thanks to Germano Cavalcante
(@mano-wii): another thread could try to use looptris data after worker
one had allocated it, but before it had actually computed looptris.

So now, we use a temp 'wip' pointer to store looptris being computed
(since this is protected by a mutex, other threads will have to wait on
it, no possibility for them to double-compute the looptris here).

This should probably be backported to 2.79a if done.