Blender Git Loki

Fix: Use a minimal alignment of 8 in MEM_lockfree_mallocN_aligned

`posix_memalign` requires the `alignment` to be at least `sizeof(void *)`.
Previously, `MEM_mallocN_aligned` would simply return `NULL` if a too small
`alignment` was used. This was an OS specific issue.

The solution is to use a minimal alignment of `8` for all aligned allocations.
The unit tests have been extended to test more possible alignments (some
of which were broken before).

Reviewers: brecht

Differential Revision: https://developer.blender.org/D6660