Some work on OpenGL platform extension handling.

(Internal change) All platform extensions are now loaded through loadPlatformEXT, rather than a platform-specific name.
Renamed xext.d to glxext.d.
Renamed winext.d to wglext.d.
Cleaned up the existing wgl stuff from the recent pull request and added support for all WGL_ARB extensions.
Public imports of the platform extensions had been added in a couple of pull requests. I've changed them to private to match how other platform modules (cgl/glx/wgl) are handled.