Linking on Windows can easily run out of memory, and limiting it
to a single link operation (i.e. disabling parallelization) should
be enough to avoid this problem.
CI currently fails with "fatal error LNK1318: Unexpected PDB error; OK (0) ''"
Google tells me it might be related to hitting a memory limit. Let's try
disabling debug for now.
GTK supports webm playback, which means a backend should always be
compiled.
The ffmpeg backend however is incomplete (no audio) and as such, we
don't want people to end up with it accidentally.
Since we don't want to drag an entire gstreamer build into our ci
on MacOs or msvc, explicitly disable the gstreame media backend there.
