Add a log warning for NaN paths

Cargo.lock

diff --git a/Cargo.lock b/Cargo.lock
index fdcb551..ca6c0fa 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -3717,6 +3717,7 @@
  "bytemuck",
  "fearless_simd",
  "libm",
+ "log",
  "peniko",
  "png",
  "roxmltree",

sparse_strips/vello_common/Cargo.toml

diff --git a/sparse_strips/vello_common/Cargo.toml b/sparse_strips/vello_common/Cargo.toml
index caf7121..1d6a9ab 100644
--- a/sparse_strips/vello_common/Cargo.toml
+++ b/sparse_strips/vello_common/Cargo.toml
@@ -25,6 +25,7 @@
 skrifa = { workspace = true, optional = true }
 smallvec = { workspace = true }
 libm = { version = "0.2.15", optional = true }
+log = { workspace = true }
 
 [features]
 default = ["std", "png", "text"]

sparse_strips/vello_common/src/flatten.rs

diff --git a/sparse_strips/vello_common/src/flatten.rs b/sparse_strips/vello_common/src/flatten.rs
index f026cbc..8356a84 100644
--- a/sparse_strips/vello_common/src/flatten.rs
+++ b/sparse_strips/vello_common/src/flatten.rs
@@ -5,6 +5,7 @@
 
 use crate::kurbo::{self, Affine, BezPath, PathEl, Stroke, StrokeOpts};
 use alloc::vec::Vec;
+use log::warn;
 
 /// The flattening tolerance.
 const TOL: f64 = 0.25;
@@ -114,6 +115,8 @@
 
     // A path that contains NaN is ill-defined, so ignore it.
     if is_nan {
+        warn!("A path contains NaN, ignoring it.");
+        
         line_buf.clear();
     }
 }