Add maketx option to sharpen as we create MIP levels.

The idea is that production sometimes thinks the filtered textures look
a little too blurry and washed out (even though we know that they're
filtered almost exactly right). So with this, we optionally do a
sharpening step (via unsharp_mask) as we create MIP levels. This gooses
the high-frequency detail just a bit.  Example command line:

  maketx input.exr -sharpen 0.5 -filter unsharp-median -o texture.exr

Play with the sharpening amount (I like somewhere between 0.25 and 0.5)
and with whether you use the unsharp-median or (by leaving out the
-filter or setting it to an ordinary name like "lanczos3") whether you
use the usual Gaussian-based unsharp mask

This is all experimental! Use at your own risk.
Fix typo in recent maketx change
CHANGES
Rearrange some fmath templates to avoid some type ambiguities
Definitions of OIIO_PURE_FUNC, OIIO_CONST_FUNC, OIIO_NOTHROW, OIIO_UNUSED
simd.h: additional functionality and fixes

abs, min, max, ceil, floor, floori, xor, shift left and right,
transpose, unary minus, specializations of shuffling for SSE3, fix buggy
int reduce_add, implement float reduce_add, fixed int4(float4)
conversion, bitcasts