Interest in exposing bitmaps (EBDT) similarly to png (CBDT)?

It doesn't take much more code (but potentially a lot of renaming) to support EBDT so I wondered if there was interest in supporting it, and an api like `hb_ot_color_glyph_reference_png` for it. There are at least some comments about adding support for some of the other formats common in EBDT tables.

An ad-hoc implementation can be seen in https://github.com/kkartaltepe/harfbuzz/commit/c42c79313d22f6a1b482bde3223560aab749381c (pardon my poor naming sense). Which works well enough for pulling bitmaps from the few fonts I needed it for.

I would probably have some time over the next few months to try and make this upstreamable, add testing, etc if there is interest (and perhaps someone to answer some questions).