stb_image: A few more tweaks

Update doc comments; name all the IDCT functions _simd; fix default mem
allocation for 1/2/4-bit images in PNG decoder.

I didn't end up doing the IDCT transpose. It saves a few ops, but it wasn't
a significant speed-up on my x86/ARM test HW (<=1% for both), and getting
this right is a bit subtle - need to make sure the dequant matrix is also
correctly transposed, for example. (My test didn't do a coeff transpose
so it produced output 8x8 blocks that were transposed too; can't ship this,
but it's good enough to get realistic perf figures).

Anyway, fiddly stuff like this for a marginal perf gain didn't seem worth it, so
screw it.

(And here's hoping the merge / PR stuff works out this time.)