[cairo] Flip offsets when reversing

Fixes many of the var_colrv1 first row tests.

https://roettsch.es/var_colrv1.html
diff --git a/src/hb-cairo-utils.cc b/src/hb-cairo-utils.cc
index 5e5dadb..3466cba 100644
--- a/src/hb-cairo-utils.cc
+++ b/src/hb-cairo-utils.cc
@@ -598,6 +598,8 @@
 
     for (unsigned i = 0; i < n_stops - 1 - i; i++)
       hb_swap (stops[i], stops[n_stops - 1 - i]);
+    for (unsigned i = 0; i < n_stops; i++)
+      stops[i].offset = 1 - stops[i].offset;
   }
 
   if (n_stops > PREALLOCATED_COLOR_STOPS)