[shaders] Add 'evolutes' macro to defines

vello_shaders/shader/flatten.wgsl

diff --git a/vello_shaders/shader/flatten.wgsl b/vello_shaders/shader/flatten.wgsl
index 5a26f59..938d4dd 100644
--- a/vello_shaders/shader/flatten.wgsl
+++ b/vello_shaders/shader/flatten.wgsl
@@ -584,10 +584,7 @@
             let lowering = EulerSegLoweringParams(
                 es, transform, path_ix, t1, scale, offset, cubic_params.chord_len, tol,
             );
-// NOTE: change this to "ifndef" to disable rendering evolutes.
-#ifdef evolute
-            contour = es_seg_flatten_offset(lowering, contour, vec2(0., 1.), /*flip=*/false);
-#else
+#ifdef evolutes
             let chord_len = length(es.p1 - es.p0);
             let cusp0 = es_params_curvature(es.params, 0.) * offset + chord_len;
             let cusp1 = es_params_curvature(es.params, 1.) * offset + chord_len;
@@ -631,6 +628,8 @@
                 flatten_offset_cusp_finalize(path_ix, transform, offset, &evolute_patch, &contour);
                 contour = es_seg_flatten_offset(lowering, contour, vec2(t, 1.), /*flip=*/false);
             }
+#else
+            contour = es_seg_flatten_offset(lowering, contour, vec2(0., 1.), /*flip=*/false);
 #endif
             last_p = this_pq1.point;
             last_q = this_pq1.deriv;

vello_shaders/src/compile/mod.rs

diff --git a/vello_shaders/src/compile/mod.rs b/vello_shaders/src/compile/mod.rs
index ab11729..0e9a562 100644
--- a/vello_shaders/src/compile/mod.rs
+++ b/vello_shaders/src/compile/mod.rs
@@ -195,13 +195,14 @@
         let imports = preprocess::get_imports(shader_dir);
         let mut errors = vec![];
         let mut info = HashMap::default();
-        let defines: HashSet<_> = if cfg!(feature = "full") {
-            vec!["full".to_string()]
-        } else {
-            vec![]
-        }
-        .into_iter()
-        .collect();
+        let defines: HashSet<_> = {
+            let mut defines = vec![];
+            if cfg!(feature = "full") {
+                defines.push("full");
+            }
+            defines.push("evolutes");
+            defines.into_iter().map(|s| s.to_owned()).collect()
+        };
         for entry in shader_dir
             .read_dir()
             .expect("Can read shader import directory")