SDL_mixer is not in the standard SDL distribution. The mixing is now
done using regular SDL_Audio functions. AudioManager is converted to a
singleton and std::shared_ptrs are used for all Sound objects. Note that
for GCC "-std=c++11" might need to be added to the CMAKE_CXX_FLAGS.
Began implementing GuiFastSelect. You can see what's currently in place by holding F2 and pressing up/down.
Hopefully fixed nearest neighbor filters with GuiImage.