Fix pointer cast warning for 64-bit builds
x264.h: fix documentation

The full details of the return values of encoder_encode and encoder_headers
were mistakenly removed a while ago; re-add them.
Don't set chroma_loc_info_present_flag for non-4:2:0

The H.264 spec says it shouldn't be set in these cases.
Write 3D metadata when outputting Matroska

For when --frame-packing is set.
x86: Pass -Worphan-labels to yasm

Makes it easier to detect typos.
x86inc: free up variable name "n" in global namespace
x86inc: warn if XOP integer FMA instruction emulation is impossible

Emulation requires a temporary register if arguments 1 and 4 are the same; this
doesn't obey the semantics of the original instruction, so we can't emulate
that in x86inc.

ffmpeg has an x86util emulation for that case; I'll add it if x264's asm ever
needs it.

Also add pmacsdql emulation.
x86inc: Support arbitrary stack alignments

If the stack is known to be at least 32-byte aligned we can safely store ymm
registers on the stack without doing manual alignment.

Change ALLOC_STACK to always align the stack before allocating stack space for
consistency. Previously alignment would occur either before or after allocating
stack space depending on whether manual alignment was required or not.
x86: Minor mbtree_propagate_cost improvements

Reduce the number of registers used from 7 to 6.
Reduce the number of vector registers used by the AVX2 implementation from 8 to 7.
Multiply fps_factor by 1/256 once per frame instead of once per macroblock row.
Use mova instead of movu for dst since it's guaranteed to be aligned.
Some cosmetics.
x86: SSE2 and SSSE3 plane_copy_deinterleave_rgb

About 5.6x faster than C on Haswell.
arm: implement x264_pixel_var_8x16_neon

checkasm --bench on a cortex-a9:
var_8x16_c: 4306
var_8x16_neon: 791
arm: implement x264_pixel_var2_8x16_neon

checkasm --bench on a cortex-a9:
var2_8x16_c: 5677
var2_8x16_neon: 1421
Macroblock tree overhaul/optimization

Move the second core part of macroblock tree into an assembly function;
SIMD-optimize roughly half of it (for x86). Roughly ~25-65% faster mbtree,
depending on content.

Slightly change how mbtree handles the tradeoff between range and precision
for propagation.

Overall a slight (but mostly negligible) effect on SSIM and ~2% faster.