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: correctly check stack alignment for Atom hadamard_ac

Regression in r2265 (only affected compilers with broken stack alignment,
like ICL on win32).
weightp: improve scale/offset search, chroma

Rescale the scale factor if the offset clips. This makes weightp more effective
in fades to/from white (and an other situation that requires big offsets).

Search more than 1 scale factor and more than 1 offset, depending on --subme.

Try to find the optimal chroma denominator instead of hardcoding it.

Overall improvement: a few percent in fade-heavy clips, such as a sample from
Avatar: TLA.
x86: more AVX2 framework, AVX2 functions

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

AVX2 functions:
mc_chroma
nal_escape
intra_sad_x3_16x16
last64
ads
hpel
dct4
idct4
quant_4x4x4
quant_4x4
quant_4x4_dc
quant_8x8
SAD_X3/X4
SATD
var
SSD
test
zigzag interleave