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

src/video/os2/SDL_os2video.c

@@ -1,6 +1,6 @@
 /*
   Simple DirectMedia Layer
-  Copyright (C) 1997-2016 Sam Lantinga <slouken@libsdl.org>
+  Copyright (C) 1997-2017 Sam Lantinga <slouken@libsdl.org>
 
   This software is provided 'as-is', without any express or implied
   warranty.  In no event will the authors be held liable for any damages
@@ -428,7 +428,7 @@ static MRESULT _wmDrop(PWINDATA pWinData, PDRAGINFO pDragInfo)
 
       // Send to SDL full file name converted to UTF-8.
       pcFName = OS2_SysToUTF8( acFName );
-      SDL_SendDropFile( pcFName );
+      SDL_SendDropFile( pWinData->window, pcFName );
       SDL_free( pcFName );
 
       // Notify a source that a drag operation is complete. 
@@ -442,6 +442,8 @@ static MRESULT _wmDrop(PWINDATA pWinData, PDRAGINFO pDragInfo)
   DrgDeleteDraginfoStrHandles( pDragInfo );
   DrgFreeDraginfo( pDragInfo );
 
+  SDL_SendDropComplete( pWinData->window );
+
   return (MRESULT)FALSE;
 }