When introducing zero copy buffers/memory mapping we broke the API,
this brings back the exact same API as before for all the previously
handled cases but still raises an exception when using a context
to map buffers.

Fixes https://gitlab.freedesktop.org/gstreamer/gst-python/-/issues/40

Part-of: <https://gitlab.freedesktop.org/gstreamer/gst-python/-/merge_requests/39>