[AD] [WIN] Fix input with external windows

["Eric Botcazou" <ebotcazou@xxxxxxxxxx>]

This is the patch I was talking about in my previous message.

When I wrote the patch merging the three input threads, I over-simplified
the input subsystem on the assumption that the main window thread would
always be the same as the input thread. That's of course blatantly false
when Allegro gets attached to an external window, because in that case
window_thread == primary_thread.

Now some DirectX and Win32 API calls need to be issued by the thread which
owns the window they are referring to. This patch aims at fixing the code
which has been breaking this rule. As a consequence, the system mouse cursor
is now correctly displayed when the mouse is moving over the title bar of an
external window.

Applied to trunk.

--
Eric Botcazou
ebotcazou@xxxxxxxxxx

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