kiva.agg backend doesn't scale line widths

When rendering with the QPainter backend, or other backends like the PDF backend, scaling is applied to rendered lines, so they get wider (seed the PDF below, which the rectangle has been scaled by (2, 1) vs. the same image rendered with the agg backend.

[test.pdf](https://github.com/enthought/enable/files/4208600/test.pdf)


This inconsistency will possibly create problems as appearance may be different on different backends, or may be problematic when trying to save a report including a Chaco plot or similar.