* tta/C/convert/convert_to_plaintext.c (pending_to_text)
(stream_to_text), tta/perl/Texinfo/Convert/PlaintextNonXS.pm
(_pending_to_text, _stream_to_text): add an optional list argument to
pending_to_text and stream_to_text.  If set, put there the anchors
found in the pending texts converted to text.  Update C callers.

* tta/C/convert/convert_to_plaintext.c
(convert_to_plaintext_internal),
tta/perl/Texinfo/Convert/PlaintextNonXS.pm (_convert): collect anchors
in math elements shown as images and put them in front of the image.

* tta/C/convert/convert_to_info.c (info_format_ref): rename
name_text_checked as name_check.

* tta/perl/t/info_tests.t (anchor_in_math), tta/perl/Makefile.tres,
tta/tests/info_tex/tex_math.texi: add test with @anchor in @math and
@displaymath.  Add a similar code to tex_math.texi, changed to have
only one @anchor in displaymath, as LaTeX does not accept two labels
in the corresponding construct.