render: support shm screencopy from direct scanout

src/ifs/zwlr_screencopy_manager_v1.rs

@@ -119,7 +119,6 @@ impl ZwlrScreencopyManagerV1 {
             with_damage: Cell::new(false),
             output_link: Cell::new(None),
             buffer: Cell::new(None),
-            is_shm: Cell::new(false),
             version: self.version,
         });
         track!(self.client, frame);