Remove black quad in forward renderer

[andeplane]

Even in forward renderer, the deferred renderer final quad is rendered as a black quad. Currently this is hacked away by setting (c080dd7)

width: root.renderMode==="deferred" ? 2.0 : 0.0
            height: width

but it shouldn't be rendered at all.

[dragly]

Had to remove the fix/hack because it crashes the app on Ubuntu with the error message:

ASSERT: "w > 0.0f" in file geometries/qplanegeometry.cpp, line 57

I assume this is an assert that requires the width of the plane to be more than zero. We need a different fix for this issue.

[andeplane]

That makes sense. We should look into the good solution, but another hack would be to have a very small quad instead :p

[andeplane]

It's not really a problem, so...