faster gdal_rasterize for a large number of small geometries

[youri]

Big performances enhancement for large number of small polygons.

The main idea is to loop over the geometries instead to loop over the pixels (by the way skipping out-of-scope geometries), and to write only concerned raster blocks. Very efficient for large files compared to original method. The old algorithm can theoretically still be more efficient for very large geometries (to be checked).

The attached files are working on 1.11.1. Two options are added to the command gdal_rasterize: the first one is -cumul which make use of an hidden option of gdalrasterize alogirithm. the second one is a switch between the old alg and the new one.

I hope this will help ! (some processes simply crash without this mod)