[cairo] Lilypond uses Cairo

[Knut Petersen]

Hello all!

As of today, a Cairo-based backend has been added to Lilypond master with commit fac01f5811. It will certainly coexist with the traditional backends for some time, but the long-term goal is to extend the functionality of the new  backend to the point where the traditional backends can ultimately be 
abandoned.

In comparison, we can see that the svg files generated with Cairo are significantly larger, however, with the Cairo-based backend some problems we have with the traditional backend do not occur.

For the generated PDFs, there is no clear advantage of a backend in terms of the sizes of the generated files. The Cairo backend is faster, but the feature set of the traditional backend is larger. For some rarely used PDF features support is missing in the Cairo library. I think of file embedding, 
for example, and also of possibilities to control font embedding and font subsetting. But I will write about such details at a later time.

Thanks to all who contributed to this great library!

Knut