winhttp: Fix handling of the headers length parameter in WinHttpAddRequestHeaders and WinHttpSendRequest.

MSDN mentions a fallback to D3DBLENDOP_ADD if the selected blendop
is not supported. In theory it would be nice to write a test for that but
I don't think this is particularly important and moreover hardware
without that cap seems hard to come by (i.e. I have none)...

The hint is not supported on core profiles. It's not guaranteed to do
anything on compatibility profile either: "The interpretation of hints is
implementation dependent. An implementation may ignore them entirely."

This state was only available on D3D 7 and below (and not guaranteed to
do anything there either, according to older DX SDK docs).
If necessary we could implement it via the GLSL 1.50 "noperspective"
interpolation qualifier, which is what we need to use for the equivalent
D3D10 feature anyway.

Makes it easier to support the different names and syntax in GLSL 1.50
in the (hopefully) near future.

This updates IXAudio2, IXAudio2SourceVoice, and IXAudio2MasteringVoice
to version 2.8. New interfaces are created for breaking changes from
version 2.7. The implementation in dlls/xaudio2_7 is updated to match.