Avoid redundant media reloads (#6763)

* Avoid redundant reloads on media viewer

* Add changelog

Riot/Modules/Room/Attachements/MXKAttachmentsViewController.m

@@ -346,6 +346,11 @@
 
 - (void)displayAttachments:(NSArray*)attachmentArray focusOn:(NSString*)eventId
 {
+    if ([attachmentArray isEqualToArray:attachments] && eventId.length == 0)
+    {
+        //  neither the attachments nor the focus changed, can be ignored
+        return;
+    }
     NSString *currentAttachmentEventId = eventId;
     NSString *currentAttachmentOriginalFileName = nil;