Also add SSE2 detection for MSVC++. Detection on GCC will follow
later.

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.)

Inside decode_block, we're still sparse, and we can use that
sparsity pattern without doing extra work to discover it.

Conflicts:
	stb_image.h

Also disable VC6 automatic precompiled headers which were enabled in some projects.

rename pngsuite/part1 to pngsuite/primary;
check in pngsuite

fix incorrect img_n variable for interlaced files, caused files to be totally incorrect if forcing channel count

fix a few assert()s that weren't STBTT_asserts();
fix missing cast for C++
fix typo in C++ test compilation that prevented it from trying to compile stb_truetype

   STBTT_POINT_SIZE
   documentation for above

stb_rect_pack:
   STBRP_ASSERT