- the expando values need to be stored now that the lines are
reformattable, otherwise the old values are lost (and they depend on
context only available at the time the line is initially printed)
- the cache is collected from the special-vars evaluation code
- the cache is controlled by the textbuffer-formats code, and stored in
the text_buffer_format_rec
- completely removed the old textbuffer representation (
https://github.com/shabble/irssi-docs/wiki/Notes-256-Colour#textbuffer-encoding
)
- textbuffer-formats is an extra module, so if we unhook the signals it
should go back to the "old way" of storing pre-rendered tex
- design uses cache, original formats and list of arguments
