tdf#96687 - force validation after External Input

After external input (EndExtTextInput), the selection was marked
as invalid, but bFormatted remained true, so all the functions that
re-evaluated and marked the portion as valid again were skipped
and therefore the invalid portion was not painted.

Change-Id: Ibb2208bdaeb8ed17a4a34fa378a2fc750abf1b07
Reviewed-on: https://gerrit.libreoffice.org/20986Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Justin Luth <justin_luth@sil.org>
Reviewed-by: Tomaž Vajngerl <quikee@gmail.com>

vcl/source/edit/textview.cxx

@@ -931,6 +931,7 @@ void TextView::Command( const CommandEvent& rCEvt )
             delete mpImpl->mpTextEngine->mpIMEInfos;
             mpImpl->mpTextEngine->mpIMEInfos = nullptr;
 
+            mpImpl->mpTextEngine->TextModified();
             mpImpl->mpTextEngine->FormatAndUpdate( this );
 
             SetInsertMode( bInsertMode );