Add comment about negative lengths to cstretchengine.h

Change-Id: Ica5ecfb63d53e9a2d6c532276df0d46b4d95bf79
Reviewed-on: https://pdfium-review.googlesource.com/c/pdfium/+/82090
Auto-Submit: Tom Sepez <tsepez@chromium.org>
Commit-Queue: Lei Zhang <thestig@chromium.org>
Reviewed-by: Lei Zhang <thestig@chromium.org>
diff --git a/core/fxge/dib/cstretchengine.h b/core/fxge/dib/cstretchengine.h
index 4e406fe..79fc60b 100644
--- a/core/fxge/dib/cstretchengine.h
+++ b/core/fxge/dib/cstretchengine.h
@@ -46,6 +46,8 @@
     CWeightTable();
     ~CWeightTable();
 
+    // Accepts a negative `dest_len` argument, producing a "mirror
+    // image" of the result if `dest_len` is negative.
     bool CalculateWeights(int dest_len,
                           int dest_min,
                           int dest_max,