plug-ins: fix incorrect loading of PSP images with uncompressed channel data.

According to the PSP specifications "Each scanline in the image data is stored on a 4 byte boundary. " Based on all actual images I've seen this is incorrect. Possibly it is a leftover from an older specification.

Note with older specification I mean older than PSP 5 (internal version 3) which is the lowest version supported by Gimp and the lowest version for which a specifications is available.

Sample images, the two uncompressed ones here: https://github.com/mike-lischke/GraphicEx/tree/master/TestSuite%20original/PSP/True%20color