Compute metrics of *TeX* logos using draw and a null painter

Compute metrics of *TeX* logos using draw and a null painter

Instead of writing a metrics() method that mimics what draw() does,
re-use the draw code with a null painter.

This has the additional benefit of fixing InsetSpecialChar logo width
with EB Garamond. I have to admit that I do not know why it was too
large with the old code, but the new code is shorter/better anyway.

This code removes logowidth(). metrics() is moved to a different
place, but only the logo cases are touched.