os/2: port from SDL2-2.0.4 to SDL2-2.0.5:

changes to SDL_os2audio.c, SDL_os2video.c, os2/SDL_systhread.c in order
to accomodate SDL2-2.0.5 changes.
- audio:  WaitDone() is gone, CloseDevice() interface changes.
- events / video:  DropFile() changes:
          SDL_DROPBEGIN and SDL_DROPCOMPLETE events, window IDs for drops.
- thread: struct SDL_Thread->stacksize
3 files changed