Interface to set xheight/cap-height explicitly

[wspr]

Imagining something like [ExHeight=5pt] or [CapsHeight=10pt] as an alternative to [Scale=MatchLowercase]. From http://tex.stackexchange.com/questions/261128/scale-font-to-a-desired-x-height-em-cap-height-etc-in-fontspec.

[ghost]

That could be useful (and thanks for the link). Currently I do that via a roundabout method using fp calculations. In the context of printed fiction, which uses long passages of running text, it is helpful to be able to compare how the text appears in different fonts. Keepingthe same the ex-height is probably the most useful head-to-head comparison.