Added note for migrating SDL2 code using SDL_ThreadID()

include/SDL3/SDL_oldnames.h

@@ -651,6 +651,7 @@
 #define SDL_TLSGet SDL_GetTLS
 #define SDL_TLSSet SDL_SetTLS
 #define SDL_threadID SDL_ThreadID
+#define SDL_ThreadID() SDL_GetCurrentThreadID()
 
 /* ##SDL_timer.h */
 #define SDL_GetTicks64 SDL_GetTicks
@@ -1295,6 +1296,7 @@
 #define SDL_TLSGet SDL_TLSGet_renamed_SDL_GetTLS
 #define SDL_TLSSet SDL_TLSSet_renamed_SDL_SetTLS
 #define SDL_threadID SDL_threadID_renamed_SDL_ThreadID
+#define SDL_ThreadID() SDL_ThreadID_renamed_SDL_GetCurrentThreadID()
 
 /* ##SDL_timer.h */
 #define SDL_GetTicks64 SDL_GetTicks64_renamed_SDL_GetTicks