[gtk] testsuite: Add blend-difference test case

[Timm Bäder <baedert src gnome org>]

commit c8865459fcc549a68b49ba5ab36d1e70efeb1ef0
Author: Timm Bäder <mail baedert org>
Date:   Mon May 6 17:33:29 2019 +0200

    testsuite: Add blend-difference test case
    
    Comparing blend nodes with mode: difference

 testsuite/gsk/compare/blend-difference.node |  11 +++++++++++
 testsuite/gsk/compare/blend-difference.png  | Bin 0 -> 507 bytes
 testsuite/gsk/meson.build                   |   1 +
 3 files changed, 12 insertions(+)
---
diff --git a/testsuite/gsk/compare/blend-difference.node b/testsuite/gsk/compare/blend-difference.node
new file mode 100644
index 0000000000..aaf8eaa4c9
--- /dev/null
+++ b/testsuite/gsk/compare/blend-difference.node
@@ -0,0 +1,11 @@
+blend {
+  mode: difference;
+  bottom: color {
+    bounds: 0 0 100 100;
+    color: red;
+  }
+  top: color {
+    bounds: 50 50 100 100;
+    color: blue;
+  }
+}
diff --git a/testsuite/gsk/compare/blend-difference.png b/testsuite/gsk/compare/blend-difference.png
new file mode 100644
index 0000000000..be918ffb7b
Binary files /dev/null and b/testsuite/gsk/compare/blend-difference.png differ
diff --git a/testsuite/gsk/meson.build b/testsuite/gsk/meson.build
index 7f6fc3b3be..90eed14aa9 100644
--- a/testsuite/gsk/meson.build
+++ b/testsuite/gsk/meson.build
@@ -37,6 +37,7 @@ compare_render_tests = [
   'outset_shadow_simple',
   'shadow-in-opacity',
   'blend-normal',
+  'blend-difference',
 ]
 
 renderers = [