Change default projection from EPSG:3785 to EPSG:3857.

EPSG:3857 is understood by Proj4 and thus understood by GDAL. It is also the
projection that is used in most web apps, such as Google Maps or Microsoft
Bing. Having the default to EPSG:3785 is not good.
gdal.py: For EPSG:3857, cancel the flattening to properly align vertically.
Automated merge with https://ecometrica.kilnhg.com/Code/Public/group/gdal2mbtiles
vips.py: Added more debug information regarding tile alignment.
gdal.py: Fix a tile alignment problem.

Some times, when upscaling and slicing a partial world, the bottom TMS extents,
which is used to determine the Y position of tiles, would be wrongfully
rounded down, thus rendering the tiles exactly 1 tile downward from where
they should be on the map.

Rounding the calculations of the TMS extents before converting the floating
point values into integers fixes the problem.
gdal.py: Deal with ESRI Web Mercator projections properly.

Added two ESRI WKID in the constants file along with their equivalent EPSG
code.