latex tables: Fix rendering for grid filled merged vertical cells

[tim-nordell-nimbelink]

The table from issue #9313:

.. table::
   :class: standard nocolorrows

   +--------------------+----------------+
   |  2 rows and 2 cols | 1 row x 3 cols |
   |                    +----+-----+-----+
   |                    |  A |  B  |  C  |
   +---+----------------+----+-----+-----+
   | 1 |       2        |  3 |  4  |  5  |
   +---+----------------+----+-----+-----+

has merged vertical cells that notably needs to draw a vertical line on the left and right-hand side of the cells in the 2x2 grid area. If you add a column to the left of this:

    .. table::
       :class: standard nocolorrows

       +---+---+----------------+----------------+
       | X |  2 rows and 2 cols | 1 row x 3 cols |
       |   |                    +----+-----+-----+
       |   |                    |  A |  B  |  C  |
       +---+---+----------------+----+-----+-----+
       | Y | 1 |       2        |  3 |  4  |  5  |
       +---+---+----------------+----+-----+-----+

then the 2x2 area should not draw a column line to the left of the area.

We can fix this by adjusting the \multicolumn invocation to adjust when it emits the colspec.

Notably, this commit does not consider the other broken case where a person maybe just wants the table borders like:

    .. tabularcolumns:: |llllll|

    .. table::
       :class: standard nocolorrows

       +---+---+----------------+----------------+
       |   |  2 rows and 2 cols | 1 row x 3 cols |
       | X |                    +----+-----+-----+
       |   |                    |  A |  B  |  C  |
       +---+---+----------------+----+-----+-----+
       | Y | 1 |       2        |  3 |  4  |  5  |
       +---+---+----------------+----+-----+-----+

That has an impact in this area of the code, but to correctly consider that one has to parse out the given colspec to determine which columns have borders between them. (This line of code has a bug for this case as the colspec being emitted should be conditional if the specific column actually has it enabled, but the supporting code only determines if a vertical line is enabled anywhere rather than which columns.)

Here are the three tables mentioned prior to this fix:

And after the fix (which just fixes the second case):

Notably, the 3rd case (which is not a full grid fill) leaves extra vertical lines, and an extraneous "dot".

[jfbu]

Thank you for contribution and sorry for late. I will rebase on current master because at my locale I got a test failure

=========================== short test summary info ============================
FAILED tests/test_builders/test_build_latex.py::test_latex_labels - assert '\\caption{labeled figure}\\label{\\detokenize{index:id1}}\\label{\\...
================== 1 failed, 88 passed, 48 warnings in 21.99s ==================

which a priori is unrelated, so I need to investigate this in the proper context suitable for merging in time for 9.1.0.

[tim-nordell-nimbelink]

Thank you for contribution and sorry for late.

You're welcome, and no worries about it taking a while. Thanks for merging!