No 512px option and --resampling not working

[timautin]

Thank you for this project that I'm trying to use in order to generate mbtiles down to zoom level 2 (gdaladdo's minsize is not working for some reason so I can't get overviews smaller than 256 px).

I add to make the following adjustments in order to get an image quality as good as what I get with GDAL:

• in constants.py, set TILE_SIDE to 512. It would be great to have this as an option.
• in vips.py, update the shrink_affine & stretch methods to use the bicubic resampling method, and support it in the affine method.

Would you accept pull requests to fix these 2 problems? The second one requires some discussion, as it seems to me that the --resampling option should be used here.

Also, the return c_int.in_dll(self.libvips, 'vips__concurrency').value in vips.py's get_concurrency throws a ValueError: dlsym(0x7ff915fc8f50, vips__concurrency): symbol not found (I'm on macOS). I had to wrap it in a try/exept and return 0 in the exept (so that it then uses cpu_count()).