Implement Accelerated 2D Canvas

Implements accelered 2d canvas and makes it an setting. Accelerated 2D
canvas can especially speed up image blitting and complex transforms on
embedded devices when using a QGLWidget viewport.

Change-Id: I98a03f3c76d4e87799830171c6df101ed9908de3
Reviewed-by: Jocelyn Turcotte <jocelyn.turcotte@digia.com>