Color transform unpremult logic changes.

[lgritz]

After some deep thinking and discussion with people who know more color
than me, I have become convinced that you almost always want to make
sure to un-premultiply colors (i.e. divide by alpha) before doing color
space transformations with OCIO, and then re-multiply by alpha (to get
back to "associated alpha") immediately after the color transformation.

To support this,

• For the ImageBufAlgo color transformation functions, change the default
  value of the unpremult parameter from false to true.

• oiiotool --colorconvert and the various --ocio commands will
  also default to doing the unpremult/premult steps, unless a new optional
  modifier is used: :unpremult=0.

• oiiotool --autocc will do the unpremult/premult steps surrounding any
  automatic color transformations that it does upon input and output.

On the whole, I think this makes the situation less error prone for
casual users. Most oiiotool command lines should just use --autocc and
then not need to worry about any explicit use of --colorconvert or
--unpremult/--premult. For the rare case when more control is needed,
the individual commands and modifiers can be used.

Note that for oiiotool, --autocc is not on by default. If you want
images auto-converted to linear space and ouputs auto-converted from
linear to the desired output space, you need to use --autocc.

[lgritz]

Updated this PR with a few additions:

1. The default for now of the oiiotool colorconvert/ociostuff modifier :unpremult= is now 0. Meaning that it preserves the old behavior unless you use :unpremult=1. Some time later (but before a major release), we will change the default.

2. I've added warnings to oiiotool in cases where it can tell that you have incorrectly double-unpremultiplied.