For metas where order might be significant if multiple metas are
attached to the same buffer, so store a sequence number with the
meta when adding it to the buffer. This allows users of the meta
to make sure metas are processed in the right order.

We need a 64-bit integer for the sequence number here in the API,
a 32-bit one might overflow too easily with high packet/buffer
rates. We could do it rtp-seqnum style of course, but that's a
bit of a pain.

We could also make it so that gst_buffer_add_meta() just keeps metas in
order or rely on the order we add the metas in, but that seems too
fragile overall, when buffers (incl. metas) get merged or split.

Also add a compare function for easier sorting.

We store the seqnum in the MetaItem struct here and not in the
GstMeta struct since there's no padding in the GstMeta struct.
We could add a private struct to GstMeta before the start of
GstMeta, but that's what MetaItem effectively is implementation-
wise. We can still change this later if we want, since it's all
private.

Fixes #262