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VP9 firstpass: Bugfix when mi_col_start/end is odd

Before this patch, if mi_col_end was odd, then the for loop for 'mb_col'
was looping once LESS than it should have been.

For example, if mi_col_end = 47, then the loop was terminating when
mb_col == 23. However, the correct behavior would be to terminate  when
mb_col == 24.

The issue was introduced in:
https://chromium-review.googlesource.com/c/webm/libvpx/+/423279

This can lead to many of the stats being inaccurate, for such videos
(with mi_col_start/end having an odd value).

As an example:
Even for very static content, fp_acc_data->intercount can never reach the
same value as num_mbs. And in turn, pcnt_inter can never reach the value 1
(that is, 100%). This would lead to very static videos NOT being marked
static, and encoded like regular videos.

Note: this is just one possible effect based on observation. Other
issues are also possible based on other stats.

Improvement on some test clips:
-------------------------------
- One test clip saw a gain of -2.580% in VBR mode (and -3.153% in Q
mode). The reason for improvement: a wrongly detected scene cut was
avoided due to corrected value in 'this_frame->pcnt_inter'.
- Some very static clips correctly marked as having 100% zero motion.
This avoided addition of unncecessary alt-refs, thereby reducing the
bitrate.

BDRate (PSNR) on regular sets (VBR mode):
-----------------------------------------
lowres: 0.0
midres: -0.027 (some clips were better/worse, but I double checked that
changes were as expected, given correction in stats calculation).
hdres: 0.0

STATS_CHANGED for the types of videos described above.

Change-Id: Ifbc2c0c0815d23ec4015475680bdf8886f158dcc

--- a/vp9/encoder/vp9_firstpass.c

+++ b/vp9/encoder/vp9_firstpass.c

@@ -820,6 +820,8 @@

   VP9_COMMON *const cm = &cpi->common;

   MACROBLOCKD *const xd = &x->e_mbd;

   TileInfo tile = tile_data->tile_info;

+  const int mb_col_start = ROUND_POWER_OF_TWO(tile.mi_col_start, 1);

+  const int mb_col_end = ROUND_POWER_OF_TWO(tile.mi_col_end, 1);

   struct macroblock_plane *const p = x->plane;

   struct macroblockd_plane *const pd = xd->plane;

   const PICK_MODE_CONTEXT *ctx = &td->pc_root->none;

@@ -846,9 +848,8 @@

   assert(new_yv12 != NULL);

   assert(frame_is_intra_only(cm) || (lst_yv12 != NULL));

-  xd->mi = cm->mi_grid_visible + xd->mi_stride * (mb_row << 1) +

-           (tile.mi_col_start >> 1);

-  xd->mi[0] = cm->mi + xd->mi_stride * (mb_row << 1) + (tile.mi_col_start >> 1);

+  xd->mi = cm->mi_grid_visible + xd->mi_stride * (mb_row << 1) + mb_col_start;

+  xd->mi[0] = cm->mi + xd->mi_stride * (mb_row << 1) + mb_col_start;

   for (i = 0; i < MAX_MB_PLANE; ++i) {

     p[i].coeff = ctx->coeff_pbuf[i][1];

@@ -862,10 +863,9 @@

   uv_mb_height = 16 >> (new_yv12->y_height > new_yv12->uv_height);

   // Reset above block coeffs.

-  recon_yoffset =

-      (mb_row * recon_y_stride * 16) + (tile.mi_col_start >> 1) * 16;

-  recon_uvoffset = (mb_row * recon_uv_stride * uv_mb_height) +

-                   (tile.mi_col_start >> 1) * uv_mb_height;

+  recon_yoffset = (mb_row * recon_y_stride * 16) + mb_col_start * 16;

+  recon_uvoffset =

+      (mb_row * recon_uv_stride * uv_mb_height) + mb_col_start * uv_mb_height;

   // Set up limit values for motion vectors to prevent them extending

   // outside the UMV borders.

@@ -873,8 +873,7 @@

   x->mv_limits.row_max =

       ((cm->mb_rows - 1 - mb_row) * 16) + BORDER_MV_PIXELS_B16;

-  for (mb_col = tile.mi_col_start >> 1, c = 0; mb_col < (tile.mi_col_end >> 1);

-       ++mb_col, c++) {

+  for (mb_col = mb_col_start, c = 0; mb_col < mb_col_end; ++mb_col, c++) {

     int this_error;

     int this_intra_error;

     const int use_dc_pred = (mb_col || mb_row) && (!mb_col || !mb_row);

@@ -920,7 +919,7 @@

     x->skip_encode = 0;

     x->fp_src_pred = 0;

     // Do intra prediction based on source pixels for tile boundaries

-    if ((mb_col == (tile.mi_col_start >> 1)) && mb_col != 0) {

+    if (mb_col == mb_col_start && mb_col != 0) {

       xd->left_mi = &mi_left;

       x->fp_src_pred = 1;

     }

@@ -1310,7 +1309,7 @@

     recon_uvoffset += uv_mb_height;

     // Accumulate row level stats to the corresponding tile stats

-    if (cpi->row_mt && mb_col == (tile.mi_col_end >> 1) - 1)

+    if (cpi->row_mt && mb_col == mb_col_end - 1)

       accumulate_fp_mb_row_stat(tile_data, fp_acc_data);

     (*(cpi->row_mt_sync_write_ptr))(&tile_data->row_mt_sync, mb_row, c,