test and fix that we cover the right inputs

At head we're redoing any n<8 tail from the start,
not continuing from (n/8)*8 like we'd want.

Change-Id: I1a3d24cdffc843bbe6f3e01a163b6e3a20fdd0ca
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/220556
Reviewed-by: Brian Osman <brianosman@google.com>
Reviewed-by: Herb Derby <herb@google.com>
Commit-Queue: Mike Klein <mtklein@google.com>
2 files changed