w32_common: show the cursor in the window border

When the cursor was in the window border, it could be hidden but it
wouldn't appear again, since mpv doesn't process mouse input there.

The code used ShowCursor, which is a horrid stateful API designed for
mouseless Win16 systems that incremented or decremented a global counter
to keep track of how many applications needed to display a special
cursor (like a busy cursor.) Replace that with a simple flag, handle
WM_SETCURSOR and use SetCursor(NULL) to hide the mouse cursor, but only
when the mouse is in the client area. DefWindowProc will set the correct
cursor on the border as long as it isn't hidden with ShowCursor.

PowerPoint also uses SetCursor(NULL) to hide the cursor when showing a
presentation, so it's probably safe.

See http://blogs.msdn.com/b/oldnewthing/archive/2009/12/17/9937972.aspx