[NTG-context] new font trickery

[mf]

When I started using ConTeXt, font management gave me headaches because 
I expected fonts to be indefinitely scalable and not to be designed in 
advance at fixed sizes.

That came from using software like Word, where fonts are scalable at any 
size. Since I had no background in typesetting, I took vector, 
indefinitely scalable fonts for granted.
With that mindset a 48pt font is just a 10pt font scaled 4.8 times.

Then I knew about optical sizes, which were the norm with lead glyphs, 
because they came in heavy physical sets. And so the notion that as the 
font body gets bigger, the stroke gets thinner (so it's not just scaling).

I like this feature because it should make playing with sizes easier for 
people while lighter for machines, even though fixed sizes to be 
designed in advance enforce discipline and consistency in documents.

Massi