Make preview to closer match main view in case of active rawdenoise and certain blend modes

Up to now the raw denoise module is always disabled in the preview pipe. Under most typical conditions that's OK. However, if fancy blend modes are active, then there is a huge discrepancy to the main view - that's irritating and contradicts user expectation. As data structures are different between main view and preview for pre-demosaic modules (raw vs. downsampled RGB) there is no way to have raw denoise act fully identical in the two pixelpipes. This set of commits allows to bring the preview pipe reasonably close to the main view.