<http://udk.openoffice.org/cpp/man/spec/textconversion.html> specifies that
FLAGS_UNDEFINED_ERROR, FLAGS_MBUNDEFINED_ERROR, and FLAGS_INVALID_ERROR: "Read
past the [erroneous] code in the input buffer [...]"  But actual behavior of
rtl_convertTextToUnicode for the various rtl_TextEncoding values has been
inconsistent.  Some erroneous input (mostly single-byte UNDEFINED and INVALID
ones) has not been consumed at all, some (multi-byte MBUNDEFINED and INVALID)
has been consumed partly, and some has been consumed fully as required.

However, at least since 8dd4265b9ddbd7786b6237676909eae5b540da0e "CWS-TOOLING:
integrate CWS hb18", Custom8BitToUnicode in sw/source/filter/ww8/ww8par.cxx
appears to rely on the broken behavior of not consuming erroneous input.  (It
reads the chunk of valid input with e.g. some RTL_TEXTENCODING_MS_125x that
happens to exhibit the broken behavior of not consuming erroneous input, then
wants to try to re-read the erroneous input with RTL_TEXTENCODING_MS_1252.  For
example, opening sw/qa/core/data/ww8/pass/forcepoint50-grfanchor-1.doc triggers
that code.  For whatever reason, the am_faksas.dot attached to
<https://bz.apache.org/ooo/show_bug.cgi?id=9240#c1> "Do not show lithuanian
letter 'Š'" appears to not, or at least no longer, trigger that code.)

Therefore, it would be useful to have a mode in which rtl_convertTextToUnicode
does not consume erroneous input.  (And I plan on doing changes in
sal/osl/unx/file* that would benefit from that behavior, too.)  But changing
rtl_convertTextToUnicode to generally not consume erroneous input would not be
feasible:  If calls do not set RTL_TEXTTOUNICODE_FLAGS_FLUSH, part of an
erroneous input can already have been consumed by a previous call, so the
current call cannot undo that.

But a change that looks like it can work is to change the behavior only if
RTL_TEXTTOUNICODE_FLAGS_FLUSH is set.  In that case we can at least not consume
the part of an erroneous input that has not yet been consumed by a previous call
(which would necessarily have been done with RTL_TEXTTOUNICODE_FLAGS_FLUSH
unset).  The expecation is that code that relies on the don't-consume behavior
will do only single calls with RTL_TEXTTOUNICODE_FLAGS_FLUSH set (so reliably
not consume the complete erroneous input), while other code (which might do
calls in a loop) will not care whether erroneous input has been consumed,
anyway.  This can be considered a mild form of behavioral API CHANGE (but note
that the old implementation didn't exhibit the requested behavior anyway).

So all implementations of rtl_convertTextToUnicode for the various
rtl_TextEncoding values have been adapted to the new behavior.  The only
exceptions are ImplDummyToUnicode (sal/textenc/textcvt.cxx), which is a special
case anyway used by RTL_TEXTENCODING_DONTKNOW, and two out of three places
(marked with a "TODO" each) in ImplUTF7ToUnicode (sal/textenc/tcvtutf7.cxx),
where it is hard to retrofit the expected behaivor, and RTL_TEXTENCODING_UTF7 is
probably not relevant for the use cases relying on the don't-consume--behavior,
anyway.

Whether a similar change should be done for rtl_convertUnicodeToText can be
examined later.

Change-Id: I1ac2c4cfd99e2a0eca219f9a3855ef110b254855
Reviewed-on: https://gerrit.libreoffice.org/78584
Tested-by: Jenkins
Reviewed-by: Stephan Bergmann <sbergman@redhat.com>