Allow user_pipeline to output compressed video

This commit relaxes the requirements of the user provided source pipeline.
Previously it had to provide raw video frames whereas now stbt does
decoding itself.  This is convenient for users but the real advantage is
that it allows us to buffer more intelligently.

raw video takes up lots of memory but it's very easy to manipulate.  You
can drop any frame (`GstBuffer`) and it will have no effect on the meaning
of any previous or subsequent frames.  Compressed video is the opposite.
If you drop buffers in an h264 stream you will see artefacts until the next
I frame comes along.

stbt has some queues set to leaky as our processing can be slower than the
rate at which frames arrive.  It is usually more important that the video
frames being analysed are recent rather than consecutive so we drop frames.
With this commit we are a little more selective and ensure that we only
drop decoded frames rather than compressed ones to avoid artefacts, even
under load.  This makes the assumption that your machine can comfortable
decode video in real time.

This is required to implement smart-tv-support where we wish to operate on
decoded frames but want to avoid dropping any frames that we've done
expensive transformations on - if we have to drop frames under load we
prefer to do it as early as possible in the pipeline to waste the minimum
amount of CPU time.  With this we will be able to insert an additional
transformation pipeline between the leaky queue and stbt's algorithms.

This feature also lays the groundwork for a performance measurement mode
where you don't want to drop any frames as you want to analyse them all,
for instance to measure animation smoothness.  This should now be possible
without having to buffer (potentially GBs of) raw frames in memory.

This is still not perfect.  Ideally in the future we will be able to drop
frames even before decoding them if we can identify that dropping them will
not produce artifacts.  This could be accomplished with a parser (e.g.
h264parse) and flushing the non-leaky queue every time a non-delta frame
is inserted.  But this commit works well enough for now.

With this change (specifically the explicit support for non-live sources)
`tests/run-performance-test.sh` once again gives useful results.  It had
been broken since the port to GStreamer 1.0 - although note: it probably
doesn't make sense to compare performance across this change.

I had to modify the source pipelines in the black-screen tests because in
those cases the GStreamer pipeline now contains a decodebin where there
was none before.  So previously during caps negotiation videotestsrc would
ask down the pipeline what the prefered caps are and the appsrc at the end
would reply "'video/x-raw,format=BGR' please" so videotestsrc would send it
in that format.  The purpose of decodebin is to take any format and
provide the data in a raw format.  So now the new decodebin replys to the
videotestsrc's caps query with "'ANY' caps are fine thanks!" and
videotestsrc defaults to "video/x-raw,format=I420" which has a subsampled
chroma.  This causes problems as with subsampling edges aren't as sharp
which can cause masks to fail among other issues.  See 1f0a74c for a
related change.
stbt.py: Add transformation pipeline

The transformation pipeline allows a transformation to be made on input
video to stb-tester.  This will be useful for Smart TV support as the
source_pipeline in this case is a video from a camera pointing at a TV
but we want to transform it such that stb-tester operates on what is
being displayed on the TV screen.

It is not possible to append such a transformation to source_pipeline
itself because there is a leaky queue after the source_pipeline.  This
leaky queue exists because stbt's image processing algorithms aren't fast
enough to analyse every frame in real time.  By dropping frames
(particularly when under load) we keep up with the live source.
Unfortunately the transformations required to undistort the image of a TV
can themselves cause significant load.  This was causing a runaway effect
where the image transformations would cause load, slowing the ability for
the image processing to read frames, causing frames to be dropped and all
the work that went into creating the frames to be thrown away.

This transformation pipeline is inserted **after** the leaky queue so we
are guaranteed that every frame we transform will be processed by stbt.
As such we must be very careful what we put in the transformation pipeline:
queues (buffers) will introduce latency so should be avoided if possible.
For now it's safer to consider it for internal use only.