A WebContext leak was introduced in MR https://gitlab.freedesktop.org/gstreamer/gst-plugins-bad/-/merge_requests/2252. If we wanted one WebContext per WebView we should also unref the WebKitWebContext when destroying the WebView. This patch reintroduces the persistent WebContext, initially part of https://gitlab.freedesktop.org/gstreamer/gst-plugins-bad/-/merge_requests/1484. Fixes #1084 Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/1933>