SvgTextContent
The text an SvgImage draws, in the space that SvgImage.textContent was given (#523).
A line is the runs of glyphs along one baseline, as a <text> and its <tspan>s draw them, with a space where the runs leave a gap. A block is the lines set one under another a line apart, as a paragraph written as one <text> per line is. SVG has no paragraphs of its own, so the lines are grouped the way a reader sees them.
Rectangles keep the smaller y in KiteRectangle.bottom, as KiteStructuredText does.