Implement GIF transparency * Migrate from gif.h to giflib * Add thirdparties submodules (rlottie, libpng, giflib) for static link * Change build type to CMake (supports both static and dynamic linking with used libraries) * Add conversion tests Notice: * Changed licence from LGPL 2.1 to BSD-3-Clause (just because) * Changed option for gif, now it is NOT background image, but framerate