riff: Mark jpeg as parsed

This is needed so that auto-plugging works with avidemux and JPEG decoders that need parsed sink caps (eg rockchip mppjpegdec decoder). It also works fine with jpegdec decoder regardless.

The qtdemux element also does the same here: https://gitlab.freedesktop.org/gstreamer/gstreamer/-/blob/main/subprojects/gst-plugins-good/gst/isomp4/qtdemux.c#L14568