Port RsvgDrawingCtx to Rust
Creating this bug to keep @pborelli and myself in the loop.
I've pushed a drawing-ctx-wip
branch to the main repo; feel free to push stuff there while we finish getting this sorted out.
This has your recent squash, plus some borrow checker fixes.
I'll fix the filters next.
The hardest thing is probably going to be with_discrete_layer()
...