We can support advanced compositing modes when rendering. So, this means per-feature pixel compositing can accept custom modes. And we can support intermediate buffers per style so that effects can be applied to groups of features as they are transferred to the main canvas.
So, porter-duff modes + other common blend modes (http://www.w3.org/TR/SVGCompositing/) + any custom gimp modes we want.
This will allow for things like:
Feature level for AGG landed in 1b210ba.
Feature level for Cairo landed in b0a0ab6
Style level for AGG landed in 81566a8. No support yet for style level in Cairo - maybe in the future.