I went back to see what changes I made to my local checkout. It seems I never made as far as to implementing subpixel rendering. I think I experimented with subpixel stuff on some other medium (maybe used GIMP to do the shifting manually, I forgot). So there may still be some hope.

On the other hand, regarding the kerning problem, there's not much you can do with the current one bitmap per glyph approach. One idea is to create multiple subpixel offset versions of the glyph. It'll cost more memory though. And the offset version of the bitmap may need to have its leftmost or rightmost column overlap the neighbouring glyph, and I'm not sure if the renderer supports gray level addition or if it would just overwrite the existing column from the previous neighbour.

As for Freetype supported fonts, there is an API for enabling subpixel rendering which you can call right before rendering the glyph. I've never experimented with it.

By the way, there is already a bug report created for this: https://savannah.gnu.org/bugs/?45629