As some thread may use a different desktop from their process.

This fixes the user32 win tests, which leaks a desktop that never gets
closed. The test_shell_window test creates a new desktop, which spawns
explorer.exe process, incrementing the desktop user count to 1, then
associates the desktop to a thread, which closes it on exit.

Never the user count is incremented to 2, and closing the thread desktop
doesn't either check whether the desktop process should be terminated.

Reversely, it is possible to create a desktop, associate it with a
thread /and/ a process, and this time the desktop process would be
terminated when the process exits, although the thread may still be
using it.

Tracking the users per thread is more robust and fixes the problem as
set_thread_desktop increments the desktop user count, and thread exit
decrements it.
Signed-off-by: Rémi Bernon <rbernon@codeweavers.com>
Signed-off-by: Alexandre Julliard <julliard@winehq.org>