Documentation on cairo backend

Hi,

I'm trying to find documentation on backend API/interfaces 
(cairo_surface_backend_t) when implementing a new backend surface. 
Specifically, what functions are required and which ones use fallback as 
well as documentation on the functions and parameters. Of special 
interest, is it possible to return a custom image surface type with 
surface_acquire_dest_image (and source_image)? Or must it be 
specifically a cairo_image_surface_t instead of my own surface?

I did look at existing backend source code and did find documentation on 
various operations used in compositing, but not much else. Any 
information would be appreciated.

Thank you,

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