Encapsulates the rendering info for a single line of the display.
The single line of info may be divided into multiple regions, each rendered
by their own SequenceRenderer. It is the job of this class to cache the
information about how much space each one wants, and how much space they want
in total. A SequenceRenderer or SequencePanel that delegates rendering to
multiple child SequenceRenderer instances may want to use these objects
for storing this information about each row they are responsible for.