Support scale_factor in cairo renderer

[springmeyer]

Usage of the scale_factor is largely missing in the cairo_renderer. Originally we figured this was not critical because SVG/PDF outputs are inherently scalable. But, we really should match renderers here because we can and should: 1) cairo also does PNG output, 2) scaling fonts/halos are not predictable after the fact, and 3) while cairo contexts have a scale() method that can be used to scale things up, they are buggy: #585 and #586

[springmeyer]

progress on this in 6915d3f

[artemp]

more progress in 0eddc2b

[springmeyer]

When applying a scale_factor greater than 1 when rendering text I do not see any problems similar to what is described in #585 and #586 - basically that bug only seems to surface if the context itself is scaled.

But the scaling is not quite enough, which likely has to do with the line drawing method not being a true outline. I'm going to close this general ticket now, and have opened new specific ones: #1344 and #1343