We need different export decorators for the different libs.
For now no actual change though, just rename before the release,
and add prelude headers to define the new decorator to GST_EXPORT.
- add data pointer to GstJpegSegment and pass segment
to all parsing functions, rename accordingly
- shorten GstJpegMarkerCode enum type name to GstJpegMarker
- move function gtk-doc blurbs into .c file
- add since markers
- flesh out docs for SOF markers
https://bugzilla.gnome.org/show_bug.cgi?id=673925