How about font rendering? Would I use freetype/harfbuzz with this? They're such a bitch to build on Windows. Currently I'm using DirectWrite but having portability would be a nice bonus, as it seems that the API of this isn't worse than Direct2D.
The Firefox and Servo team are currently hard at work on Pathfinder, which should solve this for both fonts and SVG. It'll run on the GPU like WebRender which is super exciting: https://github.com/pcwalton/pathfinder - alas it's still a WIP at the moment.