patch 8.2.0857: GTK cell height can be a pixel too much

Problem:    GTK cell height can be a pixel too much.
Solution:   Subtract 3 instead of 1 when rounding. (closes #6168)
diff --git a/src/version.c b/src/version.c
index c472819..5bd8609 100644
--- a/src/version.c
+++ b/src/version.c
@@ -747,6 +747,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    857,
+/**/
     856,
 /**/
     855,