Intent to prototype: Use SkPDF as our default print-to-pdf back-end

[Emilio Cobos Álvarez]

Hi all,

In bug 2020017, I plan to start testing the SkPDF back-end on Nightly builds for print-to-pdf output.

This will allow us to eventually get rid of mozcairo (which is the current back-end). Note that GTK3 might still use system Cairo in some cases. More importantly, this also unblocks improvements on generating more accessible PDFs (bug 1657973).

Ideally this doesn't have any user-visible behavior change, but it might need some tuning. In my limited testing, the size of the generated PDFs seems reasonable, and the print quality is good, but of course there are tons of documents you can print out there... :)

This is behind the pref `print.experimental.skpdf` (and soon `print.skpdf.image-encoding-quality` for jpeg encoding quality). Please file bugs on either print quality, PDF file size, or missing functionality against bug 1372506.

Let me know if there's any concern with proceeding.

Thanks a lot,

 -- Emilio