Moves GL, minidx12, OpenAL, stblib to 3rdparty
Fixes AppVeyor and CMakeLists (travis-ci.yml doesn't need any changes)
Points directories in the VS solution files to the new ones on the
externals directory
Includes stuff with the < > syntax instead of " "
*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.