Default to a more sensible sticker size

If we do not set the width/height for stickers (which don't have any)
then the height is okay, but the message has the maximum width which
looks odd.

Instead, let's limit all stickers to 256px and it makes them look much
nicer in chat.

src/timeline/ImageComponent.qml

@@ -184,8 +184,8 @@ Item {
     MediaSizeHelper {
         id: mediaSizeHelper
         contentMaxWidth: root.maxContentWidth
-        mediaWidth: root?.mediaInfo.width ?? 0
-        mediaHeight: root?.mediaInfo.height ?? 0
+        mediaWidth: root?.mediaInfo.isSticker ? 256 : (root?.mediaInfo.width ?? 0)
+        mediaHeight: root?.mediaInfo.isSticker ? 256 : (root?.mediaInfo.height ?? 0)
     }
 
     QtObject {