Fix some problems of svg exporting

[xia0hj]

Hi,

I found some problems when using svg exporting in 2D Sketcher:

1. The exported svg is fliped, becase in svg the positive y direction is top-to-bottom. I fixed it simply by flipping svg vertically.
2. Some sketcher objects show incompletely dut to wrong width and height, i correct the viewbox args.
3. Add supports for exporting Ellipse, EllipticalArc and BezierCurve.