[gtk] Fix textview-margins reftest

[Benjamin Otte <otte src gnome org>]

commit 3c44f83909bb672047225deabd8a7e16bb095b7b
Author: Benjamin Otte <otte redhat com>
Date:   Fri Mar 29 03:42:16 2019 +0100

    Fix textview-margins reftest
    
    Draw something, otherwise the test will timeout

 testsuite/reftests/textview-margins.css | 4 ++++
 1 file changed, 4 insertions(+)
---
diff --git a/testsuite/reftests/textview-margins.css b/testsuite/reftests/textview-margins.css
index 1160e00d49..9e1fb8d3f7 100644
--- a/testsuite/reftests/textview-margins.css
+++ b/testsuite/reftests/textview-margins.css
@@ -1 +1,5 @@
 @import "reset-to-defaults.css";
+
+.background {
+  background-color: rebeccapurple;
+}