ARM: Fix bug in x264_quant_4x4x4_neon

Regression in r2273.
x86-64: cabac_block_residual assembly

RDO: ~20% faster than C
Bitstream: ~50% faster than C
1-2% faster overall, highest on preset superfast/fast/medium.
Add slice-min-mbs feature

Works in conjunction with slice-max-mbs and/or slice-max-size to avoid overly
small slices.
Useful with certain decoders that barf on extremely small slices.

If slice-min-mbs would be violated as a result of slice-max-size, x264 will
exceed slice-max-size and print a warning.
Add slices-max feature

The H.264 spec technically has limits on the number of slices per frame. x264
normally ignores this, since most use-cases that require large numbers of
slices prefer it to. However, certain decoders may break with extremely large
numbers of slices, as can occur with some slice-max-size/mbs settings.

When set, x264 will refuse to create any slices beyond the maximum number,
even if slice-max-size/mbs requires otherwise.
x86inc: fix some corner cases of SWAP

SWAP with >=3 named (rather than numbered) args
PERMUTE followed by SWAP with 2 named args
used to produce the wrong permutation
Fix undefined behavior in x264_ratecontrol_mb
Fix array overreads that caused miscompilation in gcc 4.8
x86inc: create xm# and ym#, analagous to m#

For when we want to mix simd sizes within one function.
x86: more AVX2 framework, AVX2 quant_8x8

Use 32-byte alignment instead of 16 to satisfy AVX2 using new macros.
Change the alignment of malloc.
avx2 idct 16x16

I don't plan to actually push this (I'll probably convert it to 8x8 because
we almost never use 16x16).
I also need to benchmark this (it's not certain to be faster), so I need
real hardware.