READ-BDF should be able to build a single FONTDESCRIPTOR from a sequence of BDF fonts

Extend READ-BDF to construct a single _composite_ font from a sequence of `BDF-FONT` instances, or `.bdf` filenames, or a mix.
Glyphs would be added sequentially as needed to _complete_ the font.
This would be instead of deferring this to the runtime font completion functionality.
The resulting fonts should be saveable with `(MEDLEYFONT.WRITE.FONT ...)`
