mpeg2enc: Lower the rank to MARGINAL

mpeg2enc: Lower the rank to MARGINAL

The rationale is that it can't be properly used right now when using
it to encode mpeg2video because of the needs-to-be-rewritten properties
and format negotiation. Other encoders will negotiate in a much saner
fashion.

One such example is that when you pick mpeg2enc for mpeg2video, the
default value for the 'format' property is "Generic MPEG-1", which is
completely wrong if downstream caps are mpeg2. The whole negotiation
code needs some serious loving before this plugin can be bumped back
up to a higher rank.