gdaladdo not producing correct overviews for greyscale+alpha image

[esok]

In gdal 1.6.1, gdaladdo is not producing the correct overviews for an image I'm working on. It works correctly with gdal 1.5.4. The image is very large and can be found at ​http://www.map.nasa.gov/esok/200906191445-goes12-fulldisk-ir4.tiff in case it cannot be attached to this ticket. I then try to run:

gdaladdo -r average 200906191445-goes12-fulldisk-ir4.tiff 2 4 8 16

Symptoms: The image appears blank at some resolutions, notably 512x512 and 256x256, where the greyscale depth goes from 8 to 1. gdal 1.5.4 works correctly and the greyscale depth remains at 8 at the smaller sizes. I used ImageMagick's "identify -verbose" utility to determine bit depths for the overview images.

[Even Rouault]

Duplicate of #3033