cap-height and x-height

[elbaro]

Hi, can you consider exposing cap-height and x-height so that users can vertically align the text using those metrics?

            RunMetrics {
                ascent: metrics.ascent,
                descent: -metrics.descent,
                leading: metrics.leading,
                underline_offset,
                underline_size,
                strikethrough_offset,
                strikethrough_size,
                line_height,
            }

For example numbers can vertically align their (baseline + cap_height) / 2.0 as the center

[nicoburns]

Yeah, we should also expose at least some of the other baselines here: https://www.w3.org/TR/css-inline-3/#baseline-types
(we probably also want to implement the actual vertical alignment in parley)