Revert "Colortable Image Source: Fixed a bug in the slow (non-vigra) render path."

This reverts commit 4b4a74f384218de633e032d7c971752fa257781f.
Revert "cast before doing vigra.colors.applyColortable"

This reverts commit 5e7c295919c3471852c61f9e2eb0c29ea6192923.