There are cases where the default of color+albedo does not actually yield useful results. T78571 is one example, volume rendering another.
For those cases one has to switch to just color as input for denoising to get correct results. While this was possible to do for final frame rendering (in the layer settings), viewport denoising always used a fixed color+albedo.
This adds an option to change the input passes for viewport denoising too, so that one can use it in scenes that otherwise wouldn't work well with it.
Only potential problem I'm seeing is that the option may cause confusion where it's currently placed, since for final frame rendering the same option is per-layer and therefore has to be found somewhere else.
