[librsvg: 1/90] Add test for render_element_to_viewport() with a viewport not at 0, 0

[Federico Mena Quintero <federico src gnome org>]

commit 812b44587ec0d48904dd27ce61487d50172381f3
Author: Federico Mena Quintero <federico gnome org>
Date:   Fri Mar 8 09:12:22 2019 -0600

    Add test for render_element_to_viewport() with a viewport not at 0,0
    
    This fails currently.

 .../rect-48x48-offsetted-100x100-10x20.png         | Bin 0 -> 1331 bytes
 librsvg_crate/tests/render_to_viewport.rs          |  57 +++++++++++++++++++++
 2 files changed, 57 insertions(+)
---
diff --git a/librsvg_crate/tests/fixtures/rect-48x48-offsetted-100x100-10x20.png 
b/librsvg_crate/tests/fixtures/rect-48x48-offsetted-100x100-10x20.png
new file mode 100644
index 00000000..ac9bb3fb
Binary files /dev/null and b/librsvg_crate/tests/fixtures/rect-48x48-offsetted-100x100-10x20.png differ
diff --git a/librsvg_crate/tests/render_to_viewport.rs b/librsvg_crate/tests/render_to_viewport.rs
index 2ad44e14..51c51d44 100644
--- a/librsvg_crate/tests/render_to_viewport.rs
+++ b/librsvg_crate/tests/render_to_viewport.rs
@@ -76,3 +76,60 @@ fn render_to_viewport_with_different_size() {
         }
     }
 }
+
+#[test]
+fn render_to_offsetted_viewport() {
+    let svg = load_svg(
+        br#"<?xml version="1.0" encoding="UTF-8"?>
+<svg xmlns="http://www.w3.org/2000/svg"; width="48" height="48">
+  <rect x="8" y="8" width="32" height="32" fill="blue"/>
+</svg>
+"#,
+    );
+
+    let renderer = CairoRenderer::new(&svg);
+
+    let output = cairo::ImageSurface::create(cairo::Format::ARgb32, 100, 100).unwrap();
+
+    {
+        let cr = cairo::Context::new(&output);
+        renderer
+            .render_element_to_viewport(
+                &cr,
+                None,
+                &cairo::Rectangle {
+                    x: 10.0,
+                    y: 20.0,
+                    width: 48.0,
+                    height: 48.0,
+                },
+            )
+            .unwrap();
+    }
+
+    let mut output_file = File::create(Path::new("output.png")).unwrap();
+    output.write_to_png(&mut output_file).unwrap();
+
+    let output_surf = SharedImageSurface::new(output, SurfaceType::SRgb).unwrap();
+
+    let fixture_path = Path::new("tests/fixtures/rect-48x48-offsetted-100x100-10x20.png");
+    let mut fixture_file = BufReader::new(File::open(fixture_path).unwrap());
+
+    let expected = cairo::ImageSurface::create_from_png(&mut fixture_file).unwrap();
+    let expected_surf = SharedImageSurface::new(expected, SurfaceType::SRgb).unwrap();
+
+    let diff = compare_surfaces(&output_surf, &expected_surf).unwrap();
+
+    match diff {
+        BufferDiff::DifferentSizes => unreachable!("surfaces should be of the same size"),
+
+        BufferDiff::Diff(diff) => {
+            let surf = diff.surface.into_image_surface().unwrap();
+            let mut output_file = File::create(Path::new("diff.png")).unwrap();
+            surf.write_to_png(&mut output_file).unwrap();
+
+            assert_eq!(diff.num_pixels_changed, 0);
+            
+        }
+    }
+}