Render delegated renderer frames using the Qt scene graph.

This provides a stub implementation that renders quads and RenderPasses
using QSGNodes in QQuickWebEngineView.
The BackingStore code path is still supported when the delegated renderer
switch is not enabled. To use the new rendering, pass the following switches:
--enable-delegated-renderer --enable-threaded-compositing

This uses debug, plain color textures until we can fetch tiles and
textures from the ResourceProvider across OpenGL context boundaries.

Change-Id: I33ea0738dc5a326ef79c1435f75c044c42e8551f
Reviewed-by: Zeno Albisser <zeno.albisser@digia.com>