MrSID Files don't report statistics by default

[warmerdam]

MrSID files ought to report statistics if bApprox is TRUE in GetStatistics()
even if bForce is FALSE since they can be computed to cheaply from the top
overview level.

Request originally from Peng Gao.

[warmerdam]

I have commited a change to set bForce to TRUE if bApproxOK is TRUE in
MrSIDRasterBand::GetStatistics() before passing the request on to 
the lower level.  Thus as long as approxok is true, statistics will always
be returned even if force was false.  I have confirmed that statistics
are generated from the top overview level known to GDAL and that this is
very fast even for large datasets. 

The change is r10661 in trunk and r10662 in branches/1.4.  The fix should
appear in 1.4.1.