chore: Follow up sticker design

lib/pages/chat/events/message_content.dart

@@ -118,7 +118,9 @@ class MessageContent extends StatelessWidget {
                 ?.tryGet<int>('h');
             var width = maxSize;
             var height = maxSize;
-            var fit = BoxFit.cover;
+            var fit = event.messageType == MessageTypes.Sticker
+                ? BoxFit.contain
+                : BoxFit.cover;
             if (w != null && h != null) {
               fit = BoxFit.contain;
               if (w > h) {

lib/widgets/mxc_image.dart

@@ -151,8 +151,11 @@ class _MxcImageState extends State<MxcImage> {
 
   Widget placeholder(BuildContext context) =>
       widget.placeholder?.call(context) ??
-      const Center(
-        child: CircularProgressIndicator.adaptive(strokeWidth: 2),
+      Container(
+        width: widget.width,
+        height: widget.height,
+        alignment: Alignment.center,
+        child: const CircularProgressIndicator.adaptive(strokeWidth: 2),
       );
 
   @override