Add test re: rasterizing polygons with vertices aligned with cell centers.

Resolves issue #620 by adding a test that confirms that GeoTrellis rasterization behavior matches gdal's regarding polygons with vertices precisely aligned with a cell's center point.  This pull request also updates the gdaltest.sh script, which is a utility script for getting the number of cells included in a gdal rasterization, to use gdal_translate instead of gdal2xyz which was dropped in the new gdal version.