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

A manual cherry-pick of commit 3a862d3d205b85d13e2da0d6dc60d19e81fa39d7.

Change-Id: I3c19d6d9b1e70280df4db76ee831a5646345a42b
diff --git a/av1/common/reconinter.c b/av1/common/reconinter.c
index 627c002..5470b6dd 100644
--- a/av1/common/reconinter.c
+++ b/av1/common/reconinter.c
@@ -1739,7 +1739,7 @@
 
 void av1_build_inter_predictors_for_planes_single_buf(
     MACROBLOCKD *xd, BLOCK_SIZE bsize, int plane_from, int plane_to, int mi_row,
-    int mi_col, int ref, uint8_t *ext_dst[3], int ext_dst_stride[3],
+    int mi_col, int ref, uint8_t *ext_dst[], int ext_dst_stride[],
     int can_use_previous) {
   int plane;
   const int mi_x = mi_col * MI_SIZE;
@@ -1801,10 +1801,10 @@
 
 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;
   for (plane = plane_from; plane <= plane_to; ++plane) {
     const BLOCK_SIZE plane_bsize = get_plane_block_size(
diff --git a/av1/common/reconinter.h b/av1/common/reconinter.h
index 526a4a7..6796270 100644
--- a/av1/common/reconinter.h
+++ b/av1/common/reconinter.h
@@ -434,16 +434,18 @@
                             const uint8_t *intra_pred, int intra_stride);
 
 // Encoder only
+// |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 mi_row,
-    int mi_col, int ref, uint8_t *ext_dst[3], int ext_dst_stride[3],
+    int mi_col, int ref, uint8_t *ext_dst[], int ext_dst_stride[],
     int can_use_previous);
+// |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[]);
 
 void av1_jnt_comp_weight_assign(const AV1_COMMON *cm, const MB_MODE_INFO *mbmi,
                                 int order_idx, int *fwd_offset, int *bck_offset,