On export, mark the start of the first paragraph

No newline is written after \begin{document}, such that
the afterParbreak method would return false. This misleads
the code that outputs a display math in an ulem command
to emit a newline command instead of \noindent, causing
latex errors. This occurs only if the math is at the very
start of a document, without anything before it.

src/Buffer.cpp

@@ -1891,6 +1891,10 @@ void Buffer::writeLaTeXSource(otexstream & os,
 		os.texrow().start(TexRow::beginDocument());
 		os << "\\begin{document}\n";
 
+		// mark the start of a new paragraph by simulating a newline,
+		// so that os.afterParbreak() returns true at document start
+		os.lastChar('\n');
+
 		// output the parent macros
 		MacroSet::iterator it = parentMacros.begin();
 		MacroSet::iterator end = parentMacros.end();