msvfw32: Use window name to determine if window is created using MCIWndCreate.
The current version of the code incorrectly assumes that the lpszClass member of CREATESTRUCT passed with WM_CREATE will point to the same memory used for the CreateWindowEx class name parameter. MCIWND_Create uses this assumption to perform a pointer comparison on the class name to determine whether the MCI window is being created using MCIWndCreateA/W and should therefore expect a unicode path parameter.
As a side effect of commit e41c255b "win32u: Use send_message_timeout for WM_CREATE and WM_NCCREATE" the CREATESTRUCT lpszClass member started pointing to different memory, breaking the current implementation of MCIWND_Create().
This commit fixes the problem by changing MCIWndCreateA/W to use an internal window name, unlikely to be used by normal applications, which can then be checked in MCIWND_Create to determine if the MCI window is being created using MCIWndCreateA/W.