wgl: Refactor drivers to a libgallium_wgl.dll

wgl: Refactor drivers to a libgallium_wgl.dll

I'd like to support EGL on Windows, using the same architecture as Linux.
On Linux, libgallium_dri.so is a "megadriver," containing the Mesa Gallium
state tracker, plus the actual driver implementations.

Now, on Windows, libgallium_wgl.dll is a proper OpenGL ICD, and OpenGL32.dll
is just a stub DLL which implements the wgl* APIs in terms of that ICD.

This is the more "architecturally clean" way to share the state tracker
between EGL and WGL.

Reviewed By: Bill Kristiansen <billkris@microsoft.com>

Reviewed-by: Charmaine Lee >charmainel@vmware.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/12677>