[LyX/master] Fix expensive thinko: operator++ returns a reference, not a copy

[Jean-Marc Lasgouttes]

commit ec151de2cc530a9118517ac9299e30c17cb62255
Author: Jean-Marc Lasgouttes <lasgouttes at lyx.org>
Date:   Tue Nov 8 00:06:18 2022 +0100

    Fix expensive thinko: operator++ returns a reference, not a copy
    
    This accounts for 45% of the row breaking time on the (huge) paragraph
    of the MWE in bug #12598.
    
    Fixes part of bug #12598.
---
 src/TextMetrics.cpp |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)

diff --git a/src/TextMetrics.cpp b/src/TextMetrics.cpp
index 00fb217..f58efa7 100644
--- a/src/TextMetrics.cpp
+++ b/src/TextMetrics.cpp
@@ -979,7 +979,7 @@ class flexible_const_iterator {
 public:
 
 	//
-	flexible_const_iterator operator++() {
+	flexible_const_iterator & operator++() {
 		if (pile_.empty())
 			++cit_;
 		else