postproc/postprocess_template: Fix alignment

postProcess in postprocess_template.c copies a PPContext
to the stack, works with this copy and then copies
it back again. Said local copy uses a hardcoded alignment
of eight, although PPContext has alignment 32 since
cbe27006
(this commit was in anticipation of AVX2 code that never landed).
This leads to misalignment in the filter-(pp|pp1|pp2|pp3|qp)
FATE-tests which UBSan complains about. So avoid the local copy.
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>