fontmake will compile in tables stored as ttx in a UFO's data directory

See for instance [Chilanka-Regular.ufo/data/com.github.fonttools.ttx](https://github.com/smc/Chilanka/tree/master/sources/Chilanka-Regular.ufo/data/com.github.fonttools.ttx), which contains ttx dumps of the `DSIG`, `gasp` and `prep` tables, which end up in the final font.

We do not want to support the general case of this, but we may need some other mechanism to achieve whatever this is trying to achieve...

Use the linked Chilanka-Regular.ufo/data/com.github.fonttools.ttx example as the reproduction case, then trace the fontc entry point that reads UFO data and compiles tables. Done should distinguish unsupported general TTX data from any explicitly supported mechanism, with the intended behavior documented and covered by a regression test.

Written by the indexing model from the issue text.