support PGM and PPM formats
treat vertical tab and form feed characters as whitespace
Merge branch 'master' of https://github.com/kennethdmiller3/stb
Merge branch 'master' of https://github.com/nothings/stb
Merge branch 'master' of https://github.com/nothings/stb
Merge branch 'master' of https://github.com/nothings/stb
Merge branch 'master' of https://github.com/kennethdmiller3/stb
Merge branch 'master' of https://github.com/nothings/stb
Merge branch 'working' of https://github.com/nothings/stb
Merge branch 'master' of https://github.com/nothings/stb
Merge branch 'master' of https://github.com/nothings/stb
Merge branch 'master' of https://github.com/kennethdmiller3/stb
Merge branch 'master' of https://github.com/nothings/stb
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.