v4l2bufferpool: Port to bufferpool flush_start/stop method

Port the buffer pool to use the new flush_start/flush_stop virtual
methods added to GstBufferPool.

https://bugzilla.gnome.org/show_bug.cgi?id=727611
v4l2bufferpool: Copy already queued buffer

This is required as during preroll we pass the first buffer twice, hence already
queued. It is also useful, to allow filters replaying a previous rendered buffers.
This will require 1 more buffer in sink if last-sample is enabled, since the last
sample will not be the same as the currently queued buffer.

https://bugzilla.gnome.org/show_bug.cgi?id=722303
v4l2sink: Cleanup old pad alloc declaration
v4l2sink: implement gstvideosink.show_frame instead of gstbasesink.render

It allows to show preroll frame. Especially it allows to update the
frame when seeking in PAUSED state.

https://bugzilla.gnome.org/show_bug.cgi?id=722303
qtdemux: use GstFlowCombiner

Removes the common code to combining flow returns to let it be
handled by core gstutils' GstFlowCombiner

https://bugzilla.gnome.org/show_bug.cgi?id=709224
avidemux: use GstFlowCombiner

Removes flow return combination code to use the newly added GstFlowCombiner
matroskademux: use GstFlowCombiner

Use the flow combiner to have the standard combination results and avoid
repeating the same code

https://bugzilla.gnome.org/show_bug.cgi?id=709224
flvdemux: Use GstFlowCombiner

Use the flow combiner to have the standard combination results and avoid
repeating the same code

https://bugzilla.gnome.org/show_bug.cgi?id=709224
qtdemux: remove last flow return from stream struct

It is already stored on GstPad on core

https://bugzilla.gnome.org/show_bug.cgi?id=709224
avidemux: remove stream last flow return

GstPad already stores that information

https://bugzilla.gnome.org/show_bug.cgi?id=709224
souphttpsrc: Add custom sticky event to contain the HTTP request and response headers

This can be useful to e.g. get cookie information downstream.

https://bugzilla.gnome.org/show_bug.cgi?id=729707
h264depay: make sure we call handle_nal for each NAL

Call handle_nal for each NAL in the STAP-A RTP packet. This makes
sure we correctly extract the SPS and PPS.

Fixes https://bugzilla.gnome.org/show_bug.cgi?id=730999
qtdemux: upstream handles seek if fragmented and on time segment

Otherwise we can reject seeks on local files that contain fragmented-like
atoms like 'mvex'. Also improve a message log

https://bugzilla.gnome.org/show_bug.cgi?id=730722
vp8enc: Don't dereference NULL variable

CID #1139838
vp9enc: Don't dereference NULL checks

CID #1197703
matroskademux: don't leak doctype string in error code path

CID 1212145.