Photoshop PSD support

We now support the combined layer of Photoshop files (stored as layer 0
in the file). This way users can keep their files as multilayer PSD and
Blender always handle them as flat images.

For perfect alpha this requires an OpenImageIO update:
https://github.com/OpenImageIO/oiio/commit/342cc2633ff590a3bb278481c61ae798c7148361

Photoshop sample files:
https://github.com/OpenImageIO/oiio-images

Brecht has some pending fixes to push for OIIO as well, so we may as
well wait to update our libraries.

What works:
===========
* 8bit images (with or without alpha)
* 16bits images (alpha discarded)
* Photoshop files saved with 'Maximum Compatibility'
* Cycles, Blender internal,  BGE (and player)

Known limitations
(due to OIIO dependency):
=========================
* Images with less than 4 channels show a wrong thumbnail (bug may be in  OIIO)
* Packed images are not supported
* We do not write PSD files.

Note: old Blenders have support for PSD via Quicktime library. But due
to license issues this was discontinued.

Many thanks for Brecht van Lommel for reviewing the patch, suggesting
multiple improvements and to help solving the alpha issue.

git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@60621 954f8c5b-7b00-dc11-b283-0030488c597c