Remove Layer trait and simplify Canvas

wgpu/src/widget/canvas/geometry.rs

@@ -0,0 +1,15 @@
+use crate::Primitive;
+use std::sync::Arc;
+
+#[derive(Debug)]
+pub struct Geometry(Arc<Primitive>);
+
+impl Geometry {
+    pub(crate) fn from_primitive(primitive: Arc<Primitive>) -> Self {
+        Self(primitive)
+    }
+
+    pub(crate) fn into_primitive(self) -> Arc<Primitive> {
+        self.0
+    }
+}