Issues with basecore2d GraphicsContext

[corranwebster]

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

• 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 Fix the rendering of polygons in basecore2d-based backends #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.