Initial version of rescaling async readback API

This is implemented at backend-neutral level and so misses some
opportunities to reduce the number of passes in the GPU backend.

Filter quality is interpreted as:
none - single nearest neighbor resampling
low - chain of bilinear resamplings. 2x up/down except for one
step which may be smaller than 2x.
medium - same as low
high - when both scale factors are up then same as low but with bicubic
filtering rather than linear. Otherwise, same as low.

Bug: skia:8962

Change-Id: I4467636c14b802d6a0d9b5c363c1ad9e87a1a44b
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/213831
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: Brian Salomon <bsalomon@google.com>
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