`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
9c9ea37770