fix: ensure mapped range is dropped before unmapping buffer in capture (#8337)

After upgrading to wgpu v30, it requires to drop the `BufferView` return
by `get_mapped_range` before unmap the buffer. Fix it in this pr, making
screenshot event crash no more

crates/egui-wgpu/src/capture.rs

@@ -230,6 +230,7 @@ impl CaptureState {
                     ));
                 }
             }
+            drop(mapped_range);
             buffer.unmap();
 
             tx.send((