Added encodings.h.

Fixed some warnings.

Rev: src/modules/Image/encodings/any.c:1.17
Rev: src/modules/Image/encodings/bmp.c:1.21
Rev: src/modules/Image/encodings/encodings.h:1.1
Rev: src/modules/Image/encodings/gif.c:1.51
Rev: src/modules/Image/encodings/ilbm.c:1.14
Rev: src/modules/Image/encodings/png.c:1.32
Rev: src/modules/Image/encodings/pnm.c:1.21
Rev: src/modules/Image/encodings/pvr.c:1.8
Rev: src/modules/Image/encodings/ras.c:1.6
Rev: src/modules/Image/encodings/tim.c:1.6
Rev: src/modules/Image/encodings/xwd.c:1.14