Wrong behaviour of average overview with alpha band

[Even Rouault]

Currently, if one computes an averaged overview of a RGBA dataset where there's a transition between valid data (alpha=255) and invalid data (alpha=0), the RGB components of the overview will - correctly - ignore the source pixels that have alpha=0. However the alpha values will be averaged. So the end result will be a non-premultiplied alpha RGBA overview, which is unwanted. A premultipled alpha RGBA would perhaps be better, but another acceptable solution, at least in the case where there are only alpha=0 and alpha=255, is to mask the alpha band with itself...

[Even Rouault]

trunk r27655, r27656 , branches/1.11 r27657 : "Overview: ignore alpha=0 values when compute an average overview of an alpha band; and also avoid memory errors when calling GetMaskBand?()/GetMaskFlags() after overview computation if GetMaskXXX() has been called before (#5640)"