add the GL include (back) to Base.props
use a similar technique to GLX.cpp (by Sonic) in WGL.cpp to get
wglSwapIntervalEXT without the WGLEW check
Conflicts:
Source/Core/VideoBackends/OGL/OGL.vcxproj
Source/Core/VideoBackends/OGL/OGL.vcxproj.filters
Source/VSProps/Base.props