Improvements related to image dithering and low-bit-depth output

[lgritz]

We have options that allow adding random dither to images before
output, which when converting float images to 8 bit or less, can
greatly reduce the appearance of banding artifacts. This PR makes a
number of improvements:

• When the option is used, it should apply dither any time you are
  reducing an image with > 8 bits/channel to an output with <= 8
  bits/channel. Not just from float/half, but also from uint16, for
  example. (When outputting > 8 bits per channel, there are enough
  gradations that you tend not to see banding artifacts, so dithering
  is not necessary for 16 bit output, even if coming from a float data
  source.)

• Change the actual dither we use to blue noise. The old dither just
  used a hash to add uniformly distributed random numbers to each
  pixel value before quantization, completely uncorrelated from pixel
  to pixel. Blue noise is specially constructed to still have a
  uniform distribution of the output range, but it ensures that
  spatially, similar values are distributed as far away from each
  other as possible, rather than having similar values sometimes
  clumped together. The result is a MUCH NICER appearance.

• Of course, this means the we needed to store a blue noise table in
  the library, and some access routines for it. We incorporate a
  public domain blue noise texture as a table.

• ImageBufAlgo::noise() and oiiotool --noise now take "blue" as a
  noise name to return blue noise. Also, make "white" the (preferred)
  synonym for "uniform" (to disambiguate the fact that both white and
  blue noise are uniformly distributed; the difference is that white
  is independently random for each pixel, whereas blue shapes the
  spectrum to favor high frequencies, making it better suited for
  dithering and sampling purposes.

• ImageBufAlgo::bluenoise_image() returns a reference to a stored
  periodic blue noise image.

• oiiotool -d would let you say "uint10" or "uint12" to indicate 10 or
  12 bit output (for formats that support it), but had no way to ask
  for less than 8 bit output (for example, TIFF supports that). So we
  add "uint6", "uint4", "uint2", "uint1" as recognized names for the
  -d option in those rare cases when you specifically want to
  produce a low bit depth file. And furthermore, if you ask for
  --dither when using lower bit depths, the dither amplitude is
  adjusted to be right sized for the bit depth you are using.

• Many format output modules that support 8 bit output honor the
  option to dither the output, but in the "built-in plugins" chapter
  of the docs, most did not list this option in the section that
  describes which special output options it knows. So now we've added
  this to the docs for all the output plugins that did support it.