playsink: Fix the block diagram of deinterlace bin.

Fixes https://bugzilla.gnome.org/show_bug.cgi?id=702465
audiobasesrc: add 2 missing gst_buffer_unmap () calls

There are 2 missing calls to gst_buffer_unmap () in the error handling in
create ().

Fixes https://bugzilla.gnome.org/show_bug.cgi?id=702467
rtpbuffer: add gst_rtp_buffer_get_payload_bytes

The function gst_rtp_buffer_get_payload can not be used in Python
because it lacks necessary length parameter. This patch adds a new
function, gst_rtp_buffer_get_payload_bytes, to use from Python
bindings. The new function has the advisory "Rename to:" annotation
so it can replace the gst_rtp_buffer_get_payload whan creating
bindings.

The function gst_rtp_buffer_get_extension_bytes is also added. It wraps
gst_rtp_buffer_get_extension_data which doesn't work in Python due to
incomplete annotation and because it returns the length as number of
32-bit words.

https://bugzilla.gnome.org/show_bug.cgi?id=698562
xmptag: More efficient GSList usage

Instead of constantly appending (which gets more and more expensive), just
prepend to the list (O(1)) and reverse the list before usage.

https://bugzilla.gnome.org/show_bug.cgi?id=702545
audioencoder: unref before memset

Unref allocator and input_caps in encoder context before memsetting the
context.
rtsp: manage writer child source better

Only add the write child source when we have something to write or else
we will dispatch forever without doing anything.