stbt tv: Allow complex source pipelines with shell escaping

Previously if you had a stbt.conf like:

    source_pipeline=mysrc property="prop with spaces"

bash would get confused and would pass `property="prop` as `$2` and
separately pass `with` as `$3` and `spaces"` as `$4`.  Using `eval` fixes
this issue although it does make me slightly worried that if some bash
syntax inadvertently ended up in source_pipeline stbt tv could behave
oddly.

This is required for smart-tv support as, due to the complexities of
image processing `source_pipeline`s can require many complex parameters.
debugging: Add checking to gst_to_opencv

OpenCV requires video frames in BGR format.  Previously if you passed a
`GstSample` in another format (e.g. I420 or RGB) `gst_to_opencv` would
either produce a cryptic error message or silently continue having done
the wrong thing.

This commit fixes that by adding an assertion.  Admittedly you have to try
pretty hard to get your hands on a non BGR format `GstSample` but I've
managed it in the past and this would have saved me some debugging effort.