Lighting effects ignores alpha channel for bump map

Submitted by Raphaël Quinet (@raphael)

Link to original bug (#359610)

Description

The lighting effects plug-in (Filters->Light and Shadow->Lighting Effects) does not use the alpha channel of the bump map layer correctly.

If you enable the bump map option in this plug-in and if you select a layer that contains partially transparent areas for the bump map, then these areas will not be mapped correctly and the (arbitrary) contents of the fully transparent pixels will be used as if these pixels were opaque.

Version: git master