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.

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

For some reason Microsoft CL compiler for ARM is unable to compile malloc when parameter is an expression. malloc(x * y) will cause internal compiler error, malloc(x) is however fine.

switch from fopen to fopen_s on later MSVCs

Proper pointer check before deleting

corrected annoying warnings about 'missing initializer'. The memset
version is not better, but gcc like it more.