Fix casts in a bunch of inline functions to maintain correct const-ness

Fix casts in a bunch of inline functions to maintain correct const-ness

Make code including GStreamer headers compile with -Wcast-qual by
maintaining const-ness when casting. Also fix function signature of
gst_byte_writer_set_pos(): the byte writer should not be marked as
const.

https://bugzilla.gnome.org/show_bug.cgi?id=627910