Decouple Mesh primitives from main Primitive type

wgpu/src/primitive.rs

@@ -1,3 +1,17 @@
+use crate::core::Rectangle;
+use crate::graphics::{Damage, Mesh};
+
 pub type Primitive = crate::graphics::Primitive<Custom>;
 
-pub type Custom = ();
+#[derive(Debug, Clone, PartialEq)]
+pub enum Custom {
+    Mesh(Mesh),
+}
+
+impl Damage for Custom {
+    fn bounds(&self) -> Rectangle {
+        match self {
+            Self::Mesh(mesh) => mesh.bounds(),
+        }
+    }
+}