[librsvg] compare_to_surface(): Print how many pixels were different, and add a tolerance

[Federico Mena Quintero <federico src gnome org>]

commit c3937373410ebc891a6727a7dc7754eb906b1e89
Author: Federico Mena Quintero <federico gnome org>
Date:   Tue Mar 26 15:53:22 2019 -0600

    compare_to_surface(): Print how many pixels were different, and add a tolerance

 librsvg_crate/tests/utils/mod.rs | 12 +++++++++++-
 1 file changed, 11 insertions(+), 1 deletion(-)
---
diff --git a/librsvg_crate/tests/utils/mod.rs b/librsvg_crate/tests/utils/mod.rs
index 51c618af..d2cfbf75 100644
--- a/librsvg_crate/tests/utils/mod.rs
+++ b/librsvg_crate/tests/utils/mod.rs
@@ -117,6 +117,8 @@ pub fn compare_to_file(
     }
 }
 
+const MAX_DIFF: u8 = 2;
+
 pub fn compare_to_surface(
     output_surf: &SharedImageSurface,
     reference_surf: &SharedImageSurface,
@@ -143,7 +145,15 @@ pub fn compare_to_surface(
             let mut output_file = File::create(diff_path).unwrap();
             surf.write_to_png(&mut output_file).unwrap();
 
-            assert_eq!(diff.num_pixels_changed, 0);
+            if diff.num_pixels_changed != 0 && diff.max_diff > MAX_DIFF {
+                println!(
+                    "{}: {} pixels changed with maximum difference of {}",
+                    output_base_name,
+                    diff.num_pixels_changed,
+                    diff.max_diff,
+                );
+                unreachable!("surfaces are too different");
+            }
         }
     }
 }