Cleanup of quantization and add optional dithering on output to uint8 files.

The default is no dither -- same as it's always been.  It may be useful to
use dither when outputting to 8 bit files, to reduce banding and visible
artifacts.  Using the same dither seed on multiple images of the same
resolution will result in the same reproducible dither pattern; intentionally
using different seeds for different images (suggestion: using a hash of
the output filename) can ensure no correlation of dither between separate
images. Which choice (or whether to dither) is up to the application and may
depend on the intended use of the images.

The implementation consists of the following changes:

* Eliminate the awkward quant_* fields from ImageSpec. We never used
  them in any way other than the default conversions for each data
  format type.  A bunch of the quantization math and internal utility
  functions related to quantization can be simplified.

* New add_dither() utility function will add dither of a given amplitude
  to an array of float pixel values. Parameters include a 'dither' seed
  for the hash that determines the individual per-pixel dither amounts,
  as well as [xyz]origin parameters that place the pixels in the proper
  place in the overall dither pattern.

* Add optional dither and [xyz]origin parameters to the ImageOutput
  to_native_scanline, to_native_tile, and to_native_rectangle utility
  methods. The default is 0, which is old behavior. If dither is
  nonzero, it will use the add_dither() function to add dither prior to
  any conversion from float to uint8 (that's the only case), passing
  along the dither seed and origin values.

* All the image writers (TIFF, JPEG, etc.) that output uint8 images will
  notice a "oiio:dither" metadata, which if nonzero is interpreted as a
  request to dither float->uint8 output, using the value as the dither
  pattern seed.

* Add --dither flag to oiiotool, off by default, but when used will
  cause dithering to be added when outputting uint8 images.