Rewrite the canvas item's paint logic.

Rewrite the canvas item's paint logic.

1. Only allow the context API calls inside onPaint() slot function,
   makes the context2d handle invalid when out of scope.
2. requestPaint() method supports region parameter
3. Emit paint() signal in updatePolish() function to allow
   threaded scene graph rendering work properly with context2d api
4. Allow request painting mutiple dirty regions between 2 frames.
5. Add svgpath for context2d

Change-Id: I5ec48f7c0eb6820d5c9c16a8d0dcc0aae8d0fd2f
Reviewed-on: http://codereview.qt.nokia.com/1465
Reviewed-by: Charles Yin <charles.yin@nokia.com>