fix for associate/unassociate (and background) alpha issues for Photoshop PSD

Photoshop not only wasn't being converted to associated alpha, but also
it had the color mixed with a background.

Photoshop File Format: (you can find background at 0x1010)
http://www.adobe.com/devnet-apps/photoshop/fileformatashtml/#50577411_31265

Alpha Test Grid we made and used to calculate PS background/alpha formula from:
http://wiki.blender.org/index.php/File:Dev-PhotoshopAlphaGrid.png

In my own files (and oiio test suite) the background was either white or
unexistent. It doesn't hurt to read it from the file when available, though.

Patch done in collaboration and reviewed by Brecht van Lommel. A lot of the credit
goes to him.