api: add image_span, deprecate image_view

* `image_span<T,Rank>` is a 2D-to-4D bounded span with byte-measured
  strides (describing data layout with dimensions for channel, x, y,
  z) for us to have future APIs deal with bounded+strided regions
  rather than YOLOing raw pointers.  Among other advantages,
  image_span will have robust bounds checking in debug builds. It also
  simplifies interfaces to pass a single image_span that encapsulates
  everything about an up-to-4D data layout, rather than needing to
  pass a function a long list of individual pointers, sizes, and
  strides.

* I've templated image_span on Rank, but default to Rank=4 and am
  using that version. The templating allows future expansion to
  describe one scanline (2D) or image plane (3D) if we want to do so
  for certain API calls, but I'm not sure yet whether to bother with
  that, so for now, the ability to template by Rank is just
  flexibility for the future.

* Add image_span based versions of copy_image() and contiguize()
  utilities. I added tests and benchmarks to verify their correctness
  and that they have similar performance to the pointer based versions.

* Deprecate image_view, which we did not ever use internally to OIIO
  or in its APIs. But since we published the headers, there's a chance
  it's used downstream and would be unwise to change its behavior or
  data layout. The new image_span is what we will use moving forward
  so that we are free to modify it and start with a fresh
  slate. Keeping the definition/header for now, but marking it with a
  deprecation warning.

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

Author: lgritz

src/doc/imageioapi.rst

@@ -95,8 +95,8 @@ Efficient unique strings: ``ustring``
 
 .. _sec-span:
 
-Non-owning array views: ``span`` / ``cspan``
-============================================
+Non-owning contiguous array views: ``span`` / ``cspan``
+=======================================================
 
 .. doxygenclass:: OIIO::span
     :members:
@@ -111,6 +111,18 @@ Additionally, there is a convenience template:
 |
 
 
+.. _sec-span:
+
+Non-owning image array views: ``image_span``
+============================================
+
+.. doxygenclass:: OIIO::image_span
+    :members:
+
+
+|
+
+
 
  .. _sec-ROI: