Change signature of ComputePass::end() to self

As per review comment.
diff --git a/piet-gpu-hal/src/hub.rs b/piet-gpu-hal/src/hub.rs
index 5c7122a..6940b24 100644
--- a/piet-gpu-hal/src/hub.rs
+++ b/piet-gpu-hal/src/hub.rs
@@ -743,7 +743,7 @@
             .dispatch(pipeline, descriptor_set, workgroup_count, workgroup_size);
     }
 
-    pub unsafe fn end(&mut self) {
+    pub unsafe fn end(self) {
         self.cmd_buf.cmd_buf().end_compute_pass();
     }
 }