Build against OpenEXR 3.0 + Imath 3.0

[lgritz]

As OpenEXR works toward their 3.0 release, which includes splitting
Imath (the vector, matrix, math, and 'half' stuff) out into a separate
repo, their top of tree master finally breaks our "bleeding edge"
build.

This is a little tricky, because now our support of this dependency is
attempting to span 3 eras: 3.0 with the full split of openexr from
imath, recent openexr+ilmbase 2.x that supplies reliable exported
cmake configs, and older 2.x that predates cmake configs. Eventually
we'll phase out older versions and clean up.

Fixes in this patch generally include:

• Add a new CI test of VFX2021 + openexr/imath master. (This will go away
  when 3.0 is released and that's what we use for the main 2021 case.)

• For new enough versions of OpenEXR, use the exported config file and
  targets, rather than the old style OPENEXR_LIBRARIES style variables.
  We have kind of a mixed idiom use because we're still supporting older
  versions that predate use of the config files.

• Deal with the fact that the Imath headers now live inside
  <Imath/ImathBlah.h> rather than <OpenEXR/ImathBlah.h>. But of
  course, you need to start including things just to find out the
  version. It's tricky. The logic is to look at
  <OpenEXR/OpenEXRConfig.h> to determine the version we're dealing
  with, then include Imath stuff from one or the other depending on
  what we found. I rearranged quite a few includes to ensure that all
  this logic only happens in a couple spots and the rest gets included
  transitively. As it turns out, quite a few of our #includes of
  Imath headers were redundant.

• OpenImageIO-C depended on OpenImageIO publicly, but actually it
  should depend on OpenImageIO privately, but export its include
  needs.

• OIIO's exported config needs to look for OpenEXR.

Signed-off-by: Larry Gritz lg@larrygritz.com