A few bugs in that area were introduced by #8391. Upscaling is
supposed to perform the final blending if needed, but it didn't.
Transparency/blending is only supported by the bilinear upscaler, which
is automatically selected in that case.
The upscaling pass itself may include a final `RCAS` pass for
sharpening, in that case, blending must be performed then. So we added
the logic for that.
Fixes#9061
45f68909d7