ROOT's notion of ascent/descent is text-related:

it depends on a font + concrete string, not on a font only.
Add these special versions with also accept a string.
Mods in TText: if back-end is not TTF based,

we can still pass: the current font + the text
when calculating ascent/descent - they are not the
same for a given font in general and for a concrete string of text (that's
what TText assumes). Default versions will fallback to the old code
(without text, without font).