postproc: Made QP, nonBQP, and pQPb arrays

Also pulled QP initialization out of inner loop, which removed some redundent code.

Added some dummy fields to PPContext to allow current code to work while
changing the rest of the postprocessing code to support the arrays.

I also increased alignment requirements for some fields in the PPContext struct to
support future avx2 code.

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
