cellGifDec Module Added

*Implemented 'cellGifDec', using 'sys_fs' to access the files and the
'stb_image' (stblib) library to decode GIF buffers to Raw-pixel buffers
that can be used as textures.

*Replace 'lodepng' and 'jpeg-compressor' libraries with 'stb_image' in
cellPngDec and cellJpgDec, respectively.

*Fixed minor issues in cellPngDec and cellJpgDec.