Make sure FontForge can cleanly convert bitmap fonts to OT Bitmap format

[skef]

This is a note from a couple years ago: https://gitlab.gnome.org/GNOME/pango/-/issues/386

I thought we might already have some related reports (#4419 might be one) but maybe not many.

Basically, HarfBuzz and perhaps other shapers are reducing or eliminating their support for various bitmap font formats, but HarfBuzz at least supports bitmap fonts in an OpenType wrapper. FontForge is an obvious conversion tool but apparently things are a bit rocky.

Given our own unifont challenges noted in #4704 it would be good if FontForge could cleanly convert the more common bitmap font formats (especially BDF) to OpenType.

[ctrlcctrlv]

Are you aware of my bitmapfont2otb, @skef?

I've done quite a bit of work on this already, I think my script does it better than any existing one. I should try it on Unifont and see if it falls over, and if so, where.

[ctrlcctrlv]

clR8x8 is example bitmapfont2otb output. It has both EBDT and CFF table in .otb.

[skef]

bitmapfont2otb looks useful but I expect many bitmap font enthusiasts won't want the outlines if they can avoid them -- the whole point (from that point of view) is to ensure you're getting the precise bitmap tuned to the size in question.

[ctrlcctrlv]

I could make it optional, or users could run ttx -x CFF.