LaTeX: partially fix color issues for wrapped code lines with pagebreak

[jfbu]

This does not fix entirely #10610 but it does sufficiently for it not to
require reverting #10577 which tried to solve #8686 conundrum. In
extreme cases, the #8686 problem meant that some contents disappeared
at page bottom, so it is probably better to maintain #10577 which
will avoid any such overflow of code beyond its frame, even though in
some specific cases (a colored entity such as a long string is partly on
both pages), some syntax highlighting gets lost.

There are anyhow other issues with colors for wrapped code lines, even
with no pagebreaks involved, such as #10615. This patch does not change
the situation there.

Prior to this patch (but after #10577):

With this patch

This patch does some change to private Sphinx LaTeX macro \spx@fcolorbox and \spx@CustomFBox.

There is no CHANGES entry because it fixes issues with #10577 which has been merged but not released yet.

test done with

Test
----


.. raw:: latex

   \vspace*{15cm}%\showoutput

.. code-block:: python

   A = [x for x in range(10)]
   print("a very long line a very long line a very long line a very long line a very long line a very long line a very long line a very long line a very long line a very long line a very long line a very long line a very long line a very long line a very long line a very long line a very long line a very long line a very long line a very long line a very long line a very long line a very long line a very long line a very long line a very long line a very long line a very long line a very long line a very long line a very long line a very long line a very long line a very long line a very long line")
   A = [x for x in range(10)]


ABC

[jfbu]

complete fix came with #10648