Temporary fix for 4X8 block intra prediction.

Currently the RD loop traverses 4X8 blocks in inverted N order while
the bitstream stores blocks smaller than 8x8 in Z order. This causes a
discrepancy where the RD loop reads uninitialized data while
performing intra prediction.  As a temporary fix simply disable the
use of the extended right edge for 4X8 blocks, until the bitstream can
be changed to match the logical structure of the blocks.

Change-Id: I44a9e4fc1a15cd551a7b38c3c1227bc5dac77e9a

av1/common/reconintra.c

diff --git a/av1/common/reconintra.c b/av1/common/reconintra.c
index f573334..668892a 100644
--- a/av1/common/reconintra.c
+++ b/av1/common/reconintra.c
@@ -83,6 +83,14 @@
                          int ss_x) {
   if (!right_available) return 0;
 
+  // TODO(bshacklett, huisu): Currently the RD loop traverses 4X8 blocks in
+  // inverted N order while in the bitstream the subblocks are stored in Z
+  // order. This discrepancy makes this function incorrect when considering 4X8
+  // blocks in the RD loop, so we disable the extended right edge for these
+  // blocks. The correct solution is to change the bitstream to store these
+  // blocks in inverted N order, and then update this function appropriately.
+  if (bsize == BLOCK_4X8 && y == 1) return 0;
+
   if (y == 0) {
     int wl = mi_width_log2_lookup[bsize];
     int hl = mi_height_log2_lookup[bsize];