external/libgav1: backport prediction subsampling fix | DO NOT MERGE

cl/289984918: convolve: Use the correct subsampling for ref frames

Bug: b/147491773
Test: upstream test vectors, repro test case
Change-Id: I66773d71abe6d2a558efe9714fb927b319a8cf05
(cherry picked from commit ce9b99c2408d1d90742adad0a17e6bf56aaab1f0)
3 files changed