Add clip mode for blends

This matches the changes in the fix_blends PR.
diff --git a/piet-scene/src/scene/blend.rs b/piet-scene/src/scene/blend.rs
index 09b6e50..d6aa080 100644
--- a/piet-scene/src/scene/blend.rs
+++ b/piet-scene/src/scene/blend.rs
@@ -34,6 +34,8 @@
     Saturation = 13,
     Color = 14,
     Luminosity = 15,
+    // Clip is the same as normal, but doesn't always push a blend group.
+    Clip = 128,
 }
 
 /// Defines the layer composition function for a blend operation.
@@ -76,7 +78,7 @@
 impl Default for Blend {
     fn default() -> Self {
         Self {
-            mix: Mix::Normal,
+            mix: Mix::Clip,
             compose: Compose::SrcOver,
         }
     }