Fix new-delete type mismatch in MSDK dispatcher

[NathanGray-ChurchillNavigation]

Issue

MFXInitEx allocates dispatcher session handles as MFX_DISP_HANDLE_EX, but MFXClose releases them with delete pHandle through a MFX_DISP_HANDLE *. Since ~MFX_DISP_HANDLE is not virtual, deleting the derived object through the base pointer is undefined behavior ([expr.delete]/3). In practice this aborts any AddressSanitizer-instrumented process that opens and closes a session through the dispatcher (MSVC /fsanitize=address, similarly Clang/GCC ASan):

==66932==ERROR: AddressSanitizer: new-delete-type-mismatch on 0x12236d7a2880 in thread T0:
  object passed to delete has wrong type:
  size of the allocated type:   1072 bytes;
  size of the deallocated type: 1048 bytes.
    #0 operator delete(void *, unsigned __int64)
    #1 MFXClose libvpl/src/windows/main.cpp:609
    ...
allocated by thread T0 here:
    #0 operator new(unsigned __int64)
    #1 MFXInitEx libvpl/src/windows/main.cpp:213

We hit this in our application's ASan CI the first time it exercised MFXInitEx/MFXClose through an instrumented dispatcher.

Solution

Making ~MFX_DISP_HANDLE virtual is not viable: the passthrough functions cast the session pointer directly to struct _mfxSession * and read members assuming the base subobject is at offset zero (e.g. MFXMemory_GetSurfaceForVPP), and mfx_dispatcher.h documents that the handle layout must remain compatible. Adding a vtable pointer would shift the base subobject.

Instead, this change makes the allocation and deallocation types agree on every path:

• Allocate MFX_DISP_HANDLE_EX at the two remaining creation sites that used the base class (MFXInitEx2-style DLL-name init and AllocateCloneHandle). MFX_DISP_HANDLE_EX only extends the base with POD members (mediaAdapterType + reserved), so this is behavior-neutral for those paths.
• Spell the matching type at each delete of a base-typed pointer (MFXClose, the DLL-name init error path, and the two clone error paths). MFXInitEx's own local/HandleVector sites were already correctly typed as MFX_DISP_HANDLE_EX *.

No layout, API, or behavior changes; 6 lines total.

How Tested

• Built on Windows with MSVC (VS 2026, Ninja) from this branch: clean configure and build of libvpl.dll.
• Deployed the same patch against v2.16.0 in our product tree: with an ASan-instrumented dispatcher DLL, MFXInitEx + MFXQuery/MFXVideoENCODE_Query + MFXClose cycles (via our capability-probing code, executed per encoder/decoder node) now run to completion; unpatched they abort with the report above on the first MFXClose.
• Changed lines are clang-format clean (git clang-format reports no diff for this change).