rtpbuffer: Allow subbuffering of empty buffers

See https://bugzilla.gnome.org/show_bug.cgi?id=720162
tests: Add test for rtpbasepayload/-depayload

Fixes https://bugzilla.gnome.org/show_bug.cgi?id=720162
video: specify/restrict usage of GST_VIDEO_FORMAT_ENCODED

GST_VIDEO_FORMAT_ENCODED was added to support *extracting* video-related
information (like width, height, framerate,...) from caps.

It is __NOT__ intended to be used as a format field on video/x-raw caps.
rtpbasepayload: implement src_event function

Add a srcpad event handler and call the src_event vmethod.
rtpbasepayload: change SSRC on GstRTPCollision event

Change our SSRC and update the caps when we receive a GstRTPCollision
event from downstream.

Fixes https://bugzilla.gnome.org/show_bug.cgi?id=711560
tools: play: allow parse-launch strings for audio and video sink
videodecoder: gst_video_decoder_release_frame() is available since 1.2.2
vorbistag: Read image-type from the GstSample info struct

But for backwards compatibility keep reading it from the caps and only
use the info struct if the caps don't contain the image-type.
Some compiler warning fixes to satisfy XCode compiler

https://bugzilla.gnome.org/show_bug.cgi?id=720513
videodecoder: release buffer pool and allocator on full reset

It allows to release the buffer pool sooner (i.e. when going
to GST_STATE_READY). Previously it was released in finalize.

Fixes bug https://bugzilla.gnome.org/show_bug.cgi?id=720389
videoencoder: Release the allocator on hard resets
Revert "video: specify/restrict usage of GST_VIDEO_FORMAT_ENCODED"

This reverts commit 5fcdabd907ca45595b64131bbae0ea963e259a7c.

Instead of making it impossible to use the ENCODED format we should
just document that it must not be used for capsfeature-less caps.

Also this commit broke API/ABI.
video-format: Document usage of GST_VIDEO_FORMAT_ENCODED

This must only ever be used in caps in combination with a non-system
memory GstCapsFeatures, and where it does not make sense to specify
any of the other video formats. Examples of this would be in gst-vaapi.
descriptions: longer version of two audio codec descriptions
pbutils: add typefinder and descriptions for audio/x-xi

xi files can be read by libsndfile.
sdp: parse encryption key field

* gst-libs/gst/sdp/gstsdpmessage.c: parse encryption key field (k).

  https://bugzilla.gnome.org/show_bug.cgi?id=720215