[AD] Fix for primitives on Windows 7

["Trent Gamblin" <trent@xxxxxxxxxx>]

Here is a patch, anyone can look but I specifically wanted Paul to have a look and approve it
before it's committed. It fixes incorrect usage of custom vertex formats when using D3D, which
goes unnoticed until you run it on Windows 7. It's just a bug fix patch. It's also still possible
to create custom vertex formats for D3D, but DX is strict about how you do that. The custom vertex
format used in ex_prim is not formatted to be usable by DX, so on Windows 7 that section will
still probably crash.


Trent :{)>

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