When rendering SVGs multiple graphic contexts are used.

CanvasQt constructs a QPainter + GraphicsContext, and for
example RenderSVGText also constructs a GraphicsContext -
but the painter is shared. That leads to the problem that
the painter's end() method is called too early. Fix that.

As soon as the paintEvent() function is left, the QPainter
is destructed anyway - same affect as calling end() before.

Investigation by Rob.
Also enable antialiasing for SVG :-)