Add immutable meta.media-prefixed properties.

Add immutable meta.media-prefixed properties.

I am deprecating real_width, real_height, and source_fps in favor of new
properties prefixed by "meta.media." These are different than the
"meta.media.N.stream" and "meta.media.N.codec" properties because they
represent the selected tracks as well as some interpretation of the raw
AVFormat and AVCodec attributes in addition to reflecting "force_"
overrides.
There is still many changes to make throughout to full remove real_width
and real_height. This change just adds what melt's new auto-profile
feature needs for most use cases.