[PATCH] GDALRasterBand::ComputeStatistics() does not use arbitrary overviews

[dron]

GDALRasterBand::ComputeStatistics() does note use arbitrary overviews when computing approximate statistics and hence does not have any benefit of reduced datasets. The whole dataset will be read even if it is not required (the block sampling and AdviseRead() help a bit, but still).

The attached patch adds ability to request the subsampled overview of size ~2500 pixels and compute statistics on that reduced array.

Frank, please, take a look at the code as we talked about previously on IRC. If your are fine with this, I can add it both in trunk and in 1.5 branch. I think it can safely wait until the next maintenance release of 1.5.

Best regards, Andrey

[dron]

This is also duplicate of 2108. Sorry.