Fix block smoothing w/vert.-subsampled prog. JPEGs

The 5x5 interblock smoothing implementation, introduced in libjpeg-turbo
2.1, improperly extended the logic from the traditional 3x3 smoothing
implementation.  Both implementations point prev_block_row and
next_block_row to the current block row when processing, respectively,
the first and the last block row in the image:

  if (block_row > 0 || cinfo->output_iMCU_row > 0)
    prev_block_row =
      buffer[block_row - 1] + cinfo->master->first_MCU_col[ci];
  else
    prev_block_row = buffer_ptr;

  if (block_row < block_rows - 1 ||
      cinfo->output_iMCU_row < last_iMCU_row)
    next_block_row =
      buffer[block_row + 1] + cinfo->master->first_MCU_col[ci];
  else
    next_block_row = buffer_ptr;

6d91e950c8 naively extended that logic to
accommodate a 5x5 smoothing window:

  if (block_row > 1 || cinfo->output_iMCU_row > 1)
    prev_prev_block_row =
      buffer[block_row - 2] + cinfo->master->first_MCU_col[ci];
  else
    prev_prev_block_row = prev_block_row;

  if (block_row < block_rows - 2 ||
      cinfo->output_iMCU_row + 1 < last_iMCU_row)
    next_next_block_row =
      buffer[block_row + 2] + cinfo->master->first_MCU_col[ci];
  else
    next_next_block_row = next_block_row;

However, this new logic was only correct if block_rows == 1, so the
values of prev_prev_block_row and next_next_block_row were incorrect
when processing, respectively, the second and second to last iMCU rows
in a vertically-subsampled progressive JPEG image.

The intent was to:
- point prev_block_row to the current block row when processing the
  first block row in the image,
- point prev_prev_block_row to prev_block_row when processing the first
  two block rows in the image,
- point next_block_row to the current block row when processing the
  last block row in the image, and
- point next_next_block_row to next_block_row when processing the last
  two block rows in the image.

This commit modifies decompress_smooth_data() so that it computes the
current block row's position relative to the whole image and sets
the block row pointers based on that value.

This commit also restores a line of code that was accidentally deleted
by 6d91e950c8:

  access_rows += compptr->v_samp_factor; /* prior iMCU row too */

access_rows is merely a sanity check that tells the access_virt_barray()
method to generate an error if accessing the specified number of rows
would cause a buffer overrun.  Essentially, it is a belt-and-suspenders
measure to ensure that j*init_d_coef_controller() allocated enough rows
for the full-image virtual array.  Thus, excluding that line of code did
not cause an observable issue.

This commit also documents dbae59281f in
the change log.

Fixes #721

ChangeLog.md

@@ -7,6 +7,11 @@
 epilogue so that debuggers and profilers can reliably capture backtraces from
 within the functions.
 
+2. Fixed two minor issues in the interblock smoothing algorithm that caused
+mathematical (but not necessarily perceptible) edge block errors when
+decompressing progressive JPEG images exactly two MCU blocks in width or that
+use vertical chrominance subsampling.
+
 
 3.0.0
 =====