Strip DX9 SDK implementation so we can simplify include paths (#723)
The DXSDK contains many files which clash with the Windows SDK. As we can't necessarily control include order (without the hacky property sheet approach), we'll (unfortunately) just strip our implementation to something close to "bare minimum" (being generous - there's still a lot of excess there).
It contained DX10, DX11, etc, which has been stripped down to just DX9 (with a couple of exceptions to be safe), to avoid...