To get bounds of path, changed from StrokeExtents to PathExtents

Previously we had used g.FixedStrokeExtents with a small line width to get a bounding box around a path.  During recent testing, I was able to reproduce bug #1313773 and it was caused by g.FixedStrokeExtents() returning an empty rectangle when I attempted to crop an ellipse selection.  I believe bug #1313773 was solved by other means (I can no longer reproduce it), but I still think it would be better to use cairo_path_extents() to get the bounds of a path.