Determines widths, ascent and descent in inches.
Font lookup is performed by 'systemfonts' (so any font
registered via systemfonts::register_font(),
register_gfont(), or font_set() is found), then Cairo
computes the actual metrics. The results are accurate for
devices whose rendering finds the same font -- this is
guaranteed for 'systemfonts'-based devices (ragg, svglite,
ggiraph) and true for Cairo devices (cairo_pdf(), ...) when
the font is also installed at the system level. For devices
with their own font engine (pdf(), png(), ...) the
metrics may not match the rendering.