svg: fixed text background rectangles

- location and size looks fine now even when scaled and with larger files (used `em` for line height and `ch` for
width)
- svg width/height looks ok now, width has some problems on scaling (seems this needs to be a unit like `px` and cannot be `ch` so it's a rough estimate)
- updated liberation mono font to 2.00.5 since the current one had wrong width for the full block character (https://github.com/liberationfonts/liberation-fonts)