Stereoscopic support forces every scene to be rendered 3 times

An important aspect of the stereoscopic support was missed in the code review, unfortunately:

Creating the mono, left, right related items in the framegraph regardless of the stereoscopic-enabled setting leads to tripling the number of leaves and so leads to rendering the whole scene three times. This is wrong. The left/right eye related subtrees should not even be present at all in the framegraph when running in the default "mono" mode.

The attached screenshot shows how the barrel, with its one layer, is now rendered 3 times. Those 4 passes (mono, left, right, composition) should have been 2 (mono, composition).

The same shown in the framegraph. Having 3 QRenderTargetSelector subtrees for each layer is pretty bad.