[librsvg] render_to_viewport_with_transform(): Convert to reftest

[Federico Mena Quintero <federico src gnome org>]

commit 6b328c9664d28272dfafc9e49bd53529d90e8a52
Author: Federico Mena Quintero <federico gnome org>
Date:   Tue Mar 26 15:59:38 2019 -0600

    render_to_viewport_with_transform(): Convert to reftest

 librsvg_crate/tests/render_to_viewport.rs | 19 +++++++++++++++++--
 1 file changed, 17 insertions(+), 2 deletions(-)
---
diff --git a/librsvg_crate/tests/render_to_viewport.rs b/librsvg_crate/tests/render_to_viewport.rs
index 867880ff..2d2282ce 100644
--- a/librsvg_crate/tests/render_to_viewport.rs
+++ b/librsvg_crate/tests/render_to_viewport.rs
@@ -123,10 +123,25 @@ fn render_to_viewport_with_transform() {
     )
     .unwrap();
 
-    compare_to_file(
+    let reference_surf = cairo::ImageSurface::create(cairo::Format::ARgb32, 100, 100).unwrap();
+
+    {
+        let cr = cairo::Context::new(&reference_surf);
+
+        cr.translate(10.0, 20.0);
+        cr.translate(-10.0, -10.0);
+
+        cr.rectangle(18.0, 18.0, 32.0, 32.0);
+        cr.set_source_rgba(0.0, 0.0, 1.0, 1.0);
+        cr.fill();
+    }
+
+    let reference_surf = SharedImageSurface::new(reference_surf, SurfaceType::SRgb).unwrap();
+
+    compare_to_surface(
         &output_surf,
+        &reference_surf,
         "render_to_viewport_with_transform",
-        "rect-48x48-offsetted-100x100-10x20.png",
     );
 }