#301 PSD export with a single float channel produces garbage

[Joachim Reichel]

The attached EXR file contains the FreeImage logo converted to a single float channel. Exporting as PSD creates a file that can no longer be imported by FreeImage, and opening in Gimp shows garbage (looks as if Gimp assumes data for 3 channels, and then runs out of data).

$ ./example_import_export test_logo.exr test_logo.psd
$ ./example_import_export test_logo.psd dummy.exr
Failed to import "test_logo.psd"