Cursor (and line positioning) fix

So there turned out to be a couple things going on around lines. Fixes #967.

First off, there was a change in the line behavior when we made styling updates
that resulted in setting the various dimensions (left, top, etc.) impacting
the left-most, top-most, etc. dimension. This varied from how Shoes 3 did
things, where setting left was always setting the "point a" value, regardless
whether it was actually left. While this was perhaps more sensible, it was
a pretty big break from how things worked before.

This went in tandem with cursor relying on former Shoes 4 behavior that also
wasn't standard, specifically that Line#move would move the entire line. In
point of fact, it only moved "point a" (per the left, top setting behavior) so
even once I restored the Shoes 3 behavior, it was still wrong.

So this PR fixes both those things, aligning line movement with Shoes 3, making
sure the cursor sets all the dimensions as necessary.

Additionally I found that we were vastly complicating the painter with a
totally unnecessary transform, so I've removed that. Hurray!