Remove sort ambiguity.

Change-Id: I36eea3f58063fd5ffe441c3ed62b944abe2dcabe
Reviewed-on: https://chromium-review.googlesource.com/c/codecs/libwebp2/+/3568490
Reviewed-by: Pascal Massimino <skal@google.com>
Tested-by: WebM Builds <builds@webmproject.org>
Commit-Queue: Vincent Rabaud <vrabaud@google.com>
diff --git a/src/enc/preview/preview_opt.cc b/src/enc/preview/preview_opt.cc
index 595a947..0e017ba 100644
--- a/src/enc/preview/preview_opt.cc
+++ b/src/enc/preview/preview_opt.cc
@@ -239,7 +239,7 @@
 WP2Status PreviewData::RemoveDuplicateVertices() {
   if (vertices_.empty()) return WP2_STATUS_OK;
   // Sort by 'y' then by 'x' for easier duplicate check.
-  std::sort(vertices_.begin(), vertices_.end());
+  std::stable_sort(vertices_.begin(), vertices_.end());
 
   uint32_t last = 0;
   for (uint32_t i = 1; i < vertices_.size(); ++i) {