The viewport compositor slows down complex scenes even if it has very
simple setups. That's because it internally computes previews which
involves a fair bit of CPU computation, however, those previews are
actually never written to the original tree, so previewers weren't
really visible so it is effectively redundantly computations.
To fix this, we double down on disabling previews for the viewport
compositor and avoid any redundant computations in that case.
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