This reverts commit 12877ec3.

Frustum culling is not free, as we need to calculate the global bounds
of each mesh, and test for intersections against the frustum planes.
For scenes that you know everything is going to be rendered every frame,
it makes sense to disable this.  By default frustum culling is enabled.

This is expensive.  Instead we can notify when the global transform has
changed, and then we can return direction vectors on demand.

This reduces what we update on the backend to what actually changes

It's too expensive when we know we dont change all of the states it
resets.

qmlWarning() needs QtQml

The mat44 functions requires qdemonutils.h

...and let the z value be the distance from clipNear to the
world position.

...and let the z value be the distance from clipNear to the
world position.

Calling translateSource() on each update is expensive, so only do it
when the source changes

- Don't close main function if one is provided by the user
- Make it possible to add more then one shader per pass

Signal arguments names are public API, so it should match the property names.

Implemented by sampling the light probe texture onto a quad that fills the
entire screen, which uses an equirectangular projection.

In the future we might want to consider using separate textures, and/or
different projections. There are some minor issues with texture gradients and
equirectangular projections. These have been worked around by setting the
gradients to zero, which is not optimal, but works OK in practice. If we switch
to using texture cubes instead of equirectangular projections, these issues can
be avoided.

Otherwise we wont update the scene on the backend with the new transform
properties

We may want more fine-grained control over visibilty later, but for now
let's assume that a non-visible nodes are inactive nodes.

The default of rendering to a texture while nice for Qt Quick, is not so
performant on embedded devices where rendering to offscreen targets can
be expensive.  So for those cases, there is no a renderMode setting to
allow for the end user to specify how the view is rendered.

Texture Mode: Render scene to FBO and be a texture provider.  All Qt
Quick composition cases work, but at the const of performance and anti-
aliasing options.

RenderNode Mode: Render the scene inline, potentially clobering the 2.5d
Qt Quick renderer.

Underlay Mode: Render the scene before Qt Quick. It is possible to
render 2D content on top of the scene, but not under.

Overlay Mode: Render the scene after Qt Quick. It is possible to render
2D content under the scene, but not over.