wgl, d3d12: Handle front buffer writes for double-buffered MSAA surfaces backed by...

wgl, d3d12: Handle front buffer writes for double-buffered MSAA surfaces backed by swapchains

We don't need to go as far as the fake front thing when MSAA is being used, because the
swapchain (single-sampled) is already decoupled from the app render buffers. But we do
need to direct the frontbuffer flush to the single-sampled back buffer, and then present
the back buffer. We also need to swap the buffers when we do this, so the next blit
targets the former front buffer.

Reviewed-by: Louis-Francis Ratté-Boulianne <lfrb@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/16227>