vello_cpu: Add a `is_multi_threaded` method

diff --git a/sparse_strips/vello_cpu/src/dispatch/mod.rs b/sparse_strips/vello_cpu/src/dispatch/mod.rs
index f9830f3..e035f8e 100644
--- a/sparse_strips/vello_cpu/src/dispatch/mod.rs
+++ b/sparse_strips/vello_cpu/src/dispatch/mod.rs

@@ -92,4 +92,5 @@
         encoded_paints: &[EncodedPaint],
         image_resolver: &dyn ImageResolver,
     );
+    fn is_multi_threaded(&self) -> bool;
 }

diff --git a/sparse_strips/vello_cpu/src/dispatch/multi_threaded.rs b/sparse_strips/vello_cpu/src/dispatch/multi_threaded.rs
index a587225..473a978 100644
--- a/sparse_strips/vello_cpu/src/dispatch/multi_threaded.rs
+++ b/sparse_strips/vello_cpu/src/dispatch/multi_threaded.rs

@@ -656,6 +656,10 @@
         self.flush_tasks();
         self.clip_context.pop_clip();
     }
+
+    fn is_multi_threaded(&self) -> bool {
+        true
+    }
 }
 
 impl Debug for MultiThreadedDispatcher {

diff --git a/sparse_strips/vello_cpu/src/dispatch/single_threaded.rs b/sparse_strips/vello_cpu/src/dispatch/single_threaded.rs
index b0053ce..ce5f1db 100644
--- a/sparse_strips/vello_cpu/src/dispatch/single_threaded.rs
+++ b/sparse_strips/vello_cpu/src/dispatch/single_threaded.rs

@@ -863,6 +863,10 @@
     fn pop_clip_path(&mut self) {
         self.clip_context.pop_clip();
     }
+
+    fn is_multi_threaded(&self) -> bool {
+        false
+    }
 }
 
 /// Saves a filtered pixmap to disk for debugging purposes.

diff --git a/sparse_strips/vello_cpu/src/render.rs b/sparse_strips/vello_cpu/src/render.rs
index 2ab2e9f..734f249 100644
--- a/sparse_strips/vello_cpu/src/render.rs
+++ b/sparse_strips/vello_cpu/src/render.rs

@@ -758,6 +758,12 @@
     pub fn restore_state(&mut self, state: RenderState) {
         self.state = state;
     }
+
+    /// Whether rendering is currently configured to run in multi-threaded mode.
+    #[doc(hidden)]
+    pub fn is_multi_threaded(&self) -> bool {
+        self.dispatcher.is_multi_threaded()
+    }
 }
 
 /// Image registry implementation.