Tile sink output options (lower resolution tiles)

[manthey]

This could be considered details related to #1002.

When we write an image, we want to be able to control how downsampling is done for lower resolution layers. In vips, we have (https://www.libvips.org/API/current/VipsRegion.html)

VIPS_REGION_SHRINK_MEAN | use the average 
VIPS_REGION_SHRINK_MEDIAN | use the median 
VIPS_REGION_SHRINK_MODE | use the mode 
VIPS_REGION_SHRINK_MAX | use the maximum 
VIPS_REGION_SHRINK_MIN | use the minimum   
VIPS_REGION_SHRINK_NEAREST | use the top-left pixel 

and, in PIL we have https://pillow.readthedocs.io/en/stable/reference/Image.html#resampling-filters. PIL properly does this (see https://zuru.tech/blog/the-dangers-behind-image-resizing).

We want to add an optional kwarg: resample that would an enum that would include the PIL Resample modes and the vips options. For any of the PIL modes, we should just use PIL for downsampling. For other modes, we probably need to implement this with numpy.

Also, when downsampling, to reduce tile edge effects caused by filtering (e.g., when using Lanczos or hamming), we probably want to pick a larger area, downsample that, and crop.

[manthey]

This issue is a superset of #1486