Use TrueType font weight scale

Use TrueType font weight scale

fontconfig uses an unusual scale from 0-215 for the font weight. It
looks like it is somewhat derived from the typographic scale some font
families use, but is still rather nonstandard. Nowadays the TrueType
scale from 100-900 seems to be standard. CSS uses it, for example.

However, most importantly, VSFilter also uses the TrueType scale. So
let's use it in libass, too.