TypecastFont using `metrics.getAscentFU() - metrics.getDescentFU()` for ascent used for all undefined and whitespace Glyphs w/o a spatial outline,
leads to a potential exceeding height compared to the actual used
bounding box of the rendered text.
This in turn leads to layout issues, e.g. button labels are placed too low.

Solution:
- Whitespace/Undefined: Drop full height 'metrics.getAscentFU() - metrics.getDescentFU()', b/c of non-existing shape height.
- Otherwise, layout on AABBox or created empty shape would pick up such default hhea-table ascent which might exceed actual string height.

Sideeffect would be if user relies on height of a whitespace.
However, knowing this fact - a user may always pick the hhea-table's ascent metric values as exposed in Font and Font.Metric.

This resolves remaining layout issues, including button labels.