get_extents on other formats of cbdt

[ebraminio]

cbdt obviously needs refactoring and some parts of it may is my fault as I didn't want to touch parts I wasn't familiar. This one shouldn't be missed however, get_extents support only format 17 but reference_png apparently support others, but the only one available on NotoColorEmoji AFAICS is 17.

[behdad]

Implemented Format 18. Remains Format 19.