Expose the scene graph publically through a QSGEngine class

This change wraps QSGRenderContext and QSGContext in a new QSGEngine
class, and expose a public interface of QSGRenderer through a
QSGAbstractRenderer to make it usable on a standalone window or FBO.

Change-Id: I2d41187472424f5ea64650a006bcd61f2711f6b9
Reviewed-by: Gunnar Sletta <gunnar.sletta@jollamobile.com>
Support Canvas.antialiasing without multisample support.

FBO based rendering relies on framebuffer multisampling to do
antialiasing, which is often not available on OpenGL ES and even on
some desktop chips. As a high-level API, it is quite bad that Canvas
users (on embedded in particular) have to choose between quality
(Image) or performance (FBO).

This change implements super sampling, rendering the content
at twice the size and then scaling it down.

[ChangeLog][QtQuick][Canvas] Implement antialiasing on
FramebufferObject based render targets through super-sampling (SSAA)
when framebuffer multisampling is not available.

Change-Id: I373f3a645342dac157506b746c1e39b0f3f3f9f2
Reviewed-by: Laszlo Agocs <laszlo.agocs@digia.com>