Arrow text background size is too large

[Nicolas-Mariotti]

The size of the text background of arrows seems to take into account the formatting tags present into the text.

Example:
{signal: [
{wave: 'P..................'},
{node: 'a....b'},
{node: 'c....d'},
{node: 'e....f'}
],
edge: [
'a<->b t_frame',
'c<->d t>sub/sub<',
'e<->f tframe',
],
}

[drom]

Depend on : drom/tspan#4