Issues with basecore2d GraphicsContext

There are a number of issues with the basecore2d implementation of GraphicsContext:

- [x] a number of path methods draw sub-paths too early, rather than building up the stack of points in the path. Eg. a sequence of `move_to`, `line_to`, `arc` should produce one path not two. Fixed by #987.
- [ ] it looks like save/restore state doesn't correctly handle restoring the coordinate transform matrix to the previous state. Not entirely sure why this doesn't work - at first glance it should seem to, but it is possible there is shared state somewhere.