Tentative fix to #8159: Undo doesn't restore environment depth correctly

The undo range is extended to encompass all the paragraph after the end that
are a non-zero depth. The reason is that these paragraphs may see their depth
reduced if the last paragraph sees its depth reduced.

Note that there is a memory cost, since we store paragraphs than may not be
modified in practice. This may matter for some pathological files (people
putting all their document at depth 1 for a weird reason?)

I tried to be careful, but this can introduce new bugs. Please test.

src/Undo.cpp

@@ -23,6 +23,7 @@
 #include "DocIterator.h"
 #include "Paragraph.h"
 #include "ParagraphList.h"
+#include "ParagraphParameters.h"
 #include "Text.h"
 
 #include "mathed/MathSupport.h"
@@ -344,6 +345,15 @@ void Undo::Private::doRecordUndo(UndoKind kind,
 		advance(first, first_pit);
 		ParagraphList::const_iterator last = plist.begin();
 		advance(last, last_pit + 1);
+		// If the paragraphs after the last one have a
+		// non-zero depth and the depth of last paragraph is
+		// decremented, then these paragraphs may be affected
+		// (ticket #8159). We guard against that by saving
+		// these extra paragraphs.
+		while (last != plist.end() && last->params().depth() > 0) {
+			++last;
+			--undo.end;
+		}
 		undo.pars = new ParagraphList(first, last);
 	}