reconinter_enc: quiet -Wstringop-overflow warnings

w/gcc-11 in
  av1_build_inter_predictors_for_planes_single_buf
  av1_build_wedge_inter_predictor_from_buf

the array parameters' size may vary based on the number of planes;
replace [3] w/[] and a size comment

Change-Id: I7e9e87bffca4b67ff1e88295383dbf49f5c43a22
(cherry picked from commit 3a862d3d205b85d13e2da0d6dc60d19e81fa39d7)

av1/encoder/reconinter_enc.c

diff --git a/av1/encoder/reconinter_enc.c b/av1/encoder/reconinter_enc.c
index ca939de..5711243 100644
--- a/av1/encoder/reconinter_enc.c
+++ b/av1/encoder/reconinter_enc.c
@@ -275,7 +275,7 @@
 
 void av1_build_inter_predictors_for_planes_single_buf(
     MACROBLOCKD *xd, BLOCK_SIZE bsize, int plane_from, int plane_to, int ref,
-    uint8_t *ext_dst[3], int ext_dst_stride[3]) {
+    uint8_t *ext_dst[], int ext_dst_stride[]) {
   assert(bsize < BLOCK_SIZES_ALL);
   const MB_MODE_INFO *mi = xd->mi[0];
   const int mi_row = xd->mi_row;
@@ -402,10 +402,10 @@
 
 void av1_build_wedge_inter_predictor_from_buf(MACROBLOCKD *xd, BLOCK_SIZE bsize,
                                               int plane_from, int plane_to,
-                                              uint8_t *ext_dst0[3],
-                                              int ext_dst_stride0[3],
-                                              uint8_t *ext_dst1[3],
-                                              int ext_dst_stride1[3]) {
+                                              uint8_t *ext_dst0[],
+                                              int ext_dst_stride0[],
+                                              uint8_t *ext_dst1[],
+                                              int ext_dst_stride1[]) {
   int plane;
   assert(bsize < BLOCK_SIZES_ALL);
   for (plane = plane_from; plane <= plane_to; ++plane) {

av1/encoder/reconinter_enc.h

diff --git a/av1/encoder/reconinter_enc.h b/av1/encoder/reconinter_enc.h
index 468e32b..5d32545 100644
--- a/av1/encoder/reconinter_enc.h
+++ b/av1/encoder/reconinter_enc.h
@@ -61,16 +61,18 @@
 
 void av1_build_obmc_inter_predictors_sb(const AV1_COMMON *cm, MACROBLOCKD *xd);
 
+// |ext_dst*| are indexed from |plane_from| to |plane_to| inclusive.
 void av1_build_inter_predictors_for_planes_single_buf(
     MACROBLOCKD *xd, BLOCK_SIZE bsize, int plane_from, int plane_to, int ref,
-    uint8_t *ext_dst[3], int ext_dst_stride[3]);
+    uint8_t *ext_dst[], int ext_dst_stride[]);
 
+// |ext_dst*| are indexed from |plane_from| to |plane_to| inclusive.
 void av1_build_wedge_inter_predictor_from_buf(MACROBLOCKD *xd, BLOCK_SIZE bsize,
                                               int plane_from, int plane_to,
-                                              uint8_t *ext_dst0[3],
-                                              int ext_dst_stride0[3],
-                                              uint8_t *ext_dst1[3],
-                                              int ext_dst_stride1[3]);
+                                              uint8_t *ext_dst0[],
+                                              int ext_dst_stride0[],
+                                              uint8_t *ext_dst1[],
+                                              int ext_dst_stride1[]);
 
 #ifdef __cplusplus
 }  // extern "C"