video: Allow obtaining the nominal size of a string size

video: Allow obtaining the nominal size of a string size

At present there is a method for measuring text, but if the actual text
string is not known, it cannot be used.

For text editor we want to set the size of the entry box to cover the
expected text size. Add the concept of a 'norminal' size with a method
to calculate that for the vidconsole.

If the method is not implemented, fall back to using the font size,
which is sufficient for fixed-width fonts.

Signed-off-by: Simon Glass <sjg@chromium.org>