[Qt] Clean up fallback rendering of GraphicsContext3D to Canvas

[Qt] Clean up fallback rendering of GraphicsContext3D to Canvas
https://bugs.webkit.org/show_bug.cgi?id=78690

Reviewed by Noam Rosenthal.

Use the common paintRenderingResultsToCanvas code to retrieve the
pixels from the FBO and use a Qt port specific paintToCanvas
implementation to wrap the pixels into a QImage and render it
into the graphics context, just like it's done for the other ports.

This removes the QGraphicsObject based paint, which is an now
obsolete method of rendering.

* platform/graphics/GraphicsContext3D.h:
* platform/graphics/opengl/GraphicsContext3DOpenGLCommon.cpp:
(WebCore::GraphicsContext3D::paintRenderingResultsToCanvas):
* platform/graphics/qt/GraphicsContext3DQt.cpp:
(GraphicsContext3DPrivate):
(WebCore::GraphicsContext3D::paintToCanvas):

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@107813 268f45cc-cd09-0410-ab3c-d52691b4dbfc