[librsvg] Improve feOffset reftest

[Paolo Borelli <pborelli src gnome org>]

commit c90c96cd1d51dc4f3310be4a8a02260d6a71c77d
Author: Paolo Borelli <pborelli gnome org>
Date:   Wed Jan 1 22:54:23 2020 +0100

    Improve feOffset reftest
    
    Cover more cases: zero-offset and no-intersection

 tests/fixtures/reftests/filter-offset-ref.png | Bin 1040 -> 1274 bytes
 tests/fixtures/reftests/filter-offset.svg     |  25 ++++++++++++++++++++++---
 2 files changed, 22 insertions(+), 3 deletions(-)
---
diff --git a/tests/fixtures/reftests/filter-offset-ref.png b/tests/fixtures/reftests/filter-offset-ref.png
index ba9aeb0d..c3da529d 100644
Binary files a/tests/fixtures/reftests/filter-offset-ref.png and 
b/tests/fixtures/reftests/filter-offset-ref.png differ
diff --git a/tests/fixtures/reftests/filter-offset.svg b/tests/fixtures/reftests/filter-offset.svg
index 87b4f09a..a7c8633a 100644
--- a/tests/fixtures/reftests/filter-offset.svg
+++ b/tests/fixtures/reftests/filter-offset.svg
@@ -2,13 +2,32 @@
   viewBox="0 0 480 360" xmlns="http://www.w3.org/2000/svg";
   xmlns:xlink="http://www.w3.org/1999/xlink";>
 
-  <filter id="objectbbox" primitiveUnits="objectBoundingBox">
+  <!-- normal offset -->
+  <filter id="offset1" primitiveUnits="objectBoundingBox">
     <feOffset dx="0.5" dy="0.1"/>
   </filter>
 
+  <!-- zero offset -->
+  <filter id="offset2" primitiveUnits="objectBoundingBox">
+    <feOffset dx="0" dy="0"/>
+  </filter>
+
+  <!-- offset larger than the source (no intersection) -->
+  <filter id="offset3" primitiveUnits="objectBoundingBox">
+    <feOffset dx="100" dy="100"/>
+  </filter>
+
   <g transform="translate(30 20)">
     <rect fill="green" x="1" y="1" width="50" height="50"/>
-    <rect fill="red" x="1" y="1" width="50" height="50" filter="url(#objectbbox)"/>
-    <rect fill="red" x="1" y="57" width="50" height="50"/>
+    <rect fill="red" x="60" y="1" width="50" height="50"/>
+
+    <rect fill="green" x="1" y="60" width="50" height="50"/>
+    <rect fill="red" x="1" y="60" width="50" height="50" filter="url(#offset1)"/>
+
+    <rect fill="green" x="1" y="120" width="50" height="50"/>
+    <rect fill="red" x="1" y="120" width="50" height="50" filter="url(#offset2)"/>
+
+    <rect fill="green" x="1" y="180" width="50" height="50"/>
+    <rect fill="red" x="1" y="180" width="50" height="50" filter="url(#offset3)"/>
   </g>
 </svg>