st/wgl: allocate and resolve msaa-textures

st/wgl: allocate and resolve msaa-textures

LLVMpipe recently got the ability to render to MSAA-surfaces, but in
order for this to work on Windows, we need to allocate a separate MSAA
resource and resolve using a blit before we can display it.

Without this, we end up always displaying the first sample instead of
the resolved result.

Acked-by: Dave Airlie <airlied@redhat.com>
Reviewed-by: Roland Scheidegger <sroland@vmware.com>
Reviewed-by: Jose Fonseca <jfonseca@vmware.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4926>