viewer: add a render mode to display just the Material

crates/coremem/src/sim/mod.rs

@@ -89,6 +89,9 @@ impl<'a, R: Real, M> Sample<'a, R, M> {
     pub fn fields(&self) -> Fields<R> {
         self.fields
     }
+    pub fn material(&self) -> &'a M {
+        self.material
+    }
 
     pub fn e(&self) -> Vec3<R> {
         self.fields.e()