ukoethe/vigra — pull request #212 by cspiel (2014-10-13 14:26:49+00:00)
Title: Fix an off-by-one error in function exportImageAlpha.

A co-developer in the Enblend/Enfuse project discovered that he cannot export OpenEXR
images with an alpha channel, because a `vigra_precondition` is wrong. The problem does
not show up with image formats that support three and four channels, which is the reason
why the bug has gone unnoticed for so long.