fix giphy corner tag

lib/src/giphy_attachment.dart

@@ -55,6 +55,8 @@ class GiphyAttachment extends StatelessWidget {
             mainAxisSize: MainAxisSize.min,
             crossAxisAlignment: CrossAxisAlignment.stretch,
             children: <Widget>[
+              Stack(
+                children: [
                   Padding(
                     padding: const EdgeInsets.all(8.0),
                     child: GestureDetector(
@@ -126,6 +128,8 @@ class GiphyAttachment extends StatelessWidget {
                       ),
                     ),
                   ),
+                ],
+              ),
               if (attachment.title != null)
                 Container(
                   alignment: Alignment.bottomCenter,