Spread/choke operates as a compression of the alpha channel towards the
low bits.
Change-Id: I82aec1321b60f7f75a79e8280e761d4629f6c923
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/305183
Reviewed-by: Michael Ludwig <michaelludwig@google.com>
Commit-Queue: Florin Malita <fmalita@google.com>
