stb_image: Faster stbi__extend_receive.
stb_image: JPEG fast_ac table - decode entire AC at once.
stb_image: JPEG dequant in decode_block not IDCT.

Inside decode_block, we're still sparse, and we can use that
sparsity pattern without doing extra work to discover it.
stb_image: IDCT kernel func ptr in stbi__jpeg

I want to support SSE2 kernels that auto-detect. If implemented
using globals, this would trigger thread race checkers if stb_image
was used in multiple threads. Just make the kernels be explicitly
per-stbi__jpeg to avoid this.

(This commit is the first step in replacing existing STBI_SIMD
functionality.)
stb_image: Add more of the SSE2 skeleton.
stb_image: Add SSE2 IDCT for JPEG decoder.

Also add SSE2 detection for MSVC++. Detection on GCC will follow
later.
stb_image: Remove old installable IDCT path.
stb_image: SSE2 YCbCr->RGB kernel
stb_image: First-pass stbi__sse2_available for GCC
stb_image: Add SSE2 h2v2 resampling kernel.
stb_image: GCC fix for new SIMD stuff.
VC6:
  support using inline asm for cpuid

YCbCr:
  switch SSE code to constants that match old C;
  create C version that is same as SSE;
  tiny optimization(?) of SSE
finalize scalar YCbCr conversion back-ported from SSE version;
add missing STBIDEFs to a few functions;
update documentation with full 1.49 info even though most isn't implemented yet
STBI_MALLOC etc.
some documentation cleanup
Merge branch 'master' of https://github.com/kennethdmiller3/stb into ppm
PNM cleanup
fix a few old error messages
tweak release docs
tweak credits
tweak revision history
remove trailing whitespace
stb_image: JPEG resampler func for NEON
stb_image: NEON integer IDCT (not yet tested!)
in progress progressive stuff