external/libgav1: backport prediction border extension fix | DO NOT MERGE

cl/281117442: Fully use the frame border for reference block.

Bug: b/147491764
Test: upstream test vectors, repro test case
Change-Id: I9bc9b8490e8da1d493664145a54232414b57964c
(cherry picked from commit 22ec187ca12f72d313fe307bae3433f2719da834)
3 files changed