If windowsdriproto headers are available, build a Windows-DRI extension,
which supports requests to enable local clients to directly render GL to a
Windows drawable:

- a query to check if WGL is being used on a screen
- a query to map a fbconfigID to a native pixelformatindex
- a query to map a drawable to a native handle

Windows-DRI can only be useful if we are using WGL, so make an note if WGL
is active on a screen.

Make validGlxDrawable() public

Adjust glxWinSetPixelFormat() so it doesn't require a context, just a
screen and config.

That enables factoring out the deferred drawable creation code as
glxWinDeferredCreateDrawable()

Enhance glxWinDeferredCreateDrawable(), so that pixmaps are placed into a
file mapping, so they exist in memory which can be shared with the direct
rendering process.

Currently, this file mapping is accessed by a name generated from the XID.
This will not be unique across multiple server instances. It would perhaps
be better, although more compli...