minor bugfixes in gif encode

Rev: src/modules/Image/colortable.c:1.23
Rev: src/modules/Image/doc/Image.html:1.14
Rev: src/modules/Image/doc/Image.image.html:1.17
Rev: src/modules/Image/encodings/gif.c:1.14
Rev: src/modules/Image/image.c:1.53
Rev: src/modules/Image/togif.c:1.24