libgl-gdi: Avoid hangs on DLL_PROCESS_DETACH.

libgl-gdi: Avoid hangs on DLL_PROCESS_DETACH.

At process exit DLL_PROCESS_DETACH is signaled to DllMain(), where then
a final cleanup is triggered.  In stw_cleanup() code is triggered that
tries to communicate a shutdown to the spawned threads -- however at
that time those threads have already been terminated by the OS and so
the process hangs.

v2: skip stw_cleanup_thread() too

Signed-off-by: José Fonseca <jfonseca@vmware.com>