I was curious if cairo is in fact slower or faster than our current renderer. I didn't feel like converting us totally to cairo yet, but we have outline mode which is much simpler and perfect for tests like this. So in about 3 hours (of which 2 hours were spent figuring out a known bug in cairo) I converted our outline mode rendering to cairo.

The result of this experiment, after I adjusted the tolerance of cairo to match our outline mode quality, is near parity. That is, cairo is about as fast as our own renderer, at least for outline mode (i.e. lots of paths but no gradients or transparency). However this claim needs to be qualified:

- It is only true for the latest version of cairo (tested with 1.3.14). Versions 1.2.x are noticeably slower.

- This uses image backend to render to a memory buffer, and therefore cannot use any hardware acceleration. Using hardware-accelerated backends will require much more drastic changes in Inkscape's architecture.

- Even with image backend, currently cairo is used _only_ for path rendering, each path going to its own temp buffer, and these buffers are later composed together just as they were before. I suspect that if this is optimized by allowing a single cairo context draw all of the paths in the drawing into a single buffer, performance will improve noticeably.

In any case, since this does not make for any speed regression, and since it fixes at least one bug that our native renderer had in outline mode, I committed this. Please test outline mode thoroughly and report if you find any problems. If everything works fine I'll remove the old outline mode code and will look into doing some further optimizations.