video: Add an id to the video frame

video: Add an id to the video frame

Rename @view_id to @id.
Add an id to the video metadata. Add a method to get the metadata from a buffer
with the given id.
Make a method to map a frame with a certain id. This only maps the frame with
the given id on the video metadata. The generic frame id can be used when a
buffer carries multiple video frames such as in multiview mode but maybe also
when dealing with interlaced video that stores the fields in separate buffers.