Fix Gdal driver to use fast subsampling to create elevation layers.
FIX compilation problem on Windows. Change 'round' to 'osg::round'
Implement more accurate buffer calculations for subsampled gdal tiles.
Add comments to last code change.
Merge branch 'fastgdal' of git://github.com/rolandhill/osgearth into rolandhill-fastgdal

Conflicts:
	src/osgEarthDrivers/gdal/ReaderWriterGDAL.cpp
Merge branch 'rolandhill-fastgdal'