mpeg2_metadata, cbs_mpeg2: Fix handling of colour_description

mpeg2_metadata, cbs_mpeg2: Fix handling of colour_description

If a sequence display extension is read with colour_description equal to
zero, but a user wants to add one or more of the colour_description
elements, then the colour_description elements the user did not explicitly
request to be set are set to zero and not to the value equal to
unknown/unspecified (namely 2). A value of zero is not only inappropriate,
but explicitly forbidden. This is fixed by inferring the right default
values during the reading process if the elements are absent; moreover,
changing any of the colour_description elements to zero is now no longer
possible.

Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>