[luatex] process_output_buffer callback?

[Taco Hoekwater]

Manuel Pégourié-Gonnard wrote:
> 
> In order to help saving your time, I attach a proposed implementation. I ran
> quite a few tests with it, and the results look ok.
> 
> Please consider including this new callback in LuaTeX.

Patch applied (#2898)

Best wishes,
Taco