Dark Theme - Bug Fix

Recents List: some room avatars have an unexpected black background.
We apply here the new property `defaultBackgroundColor` defined in MXKImageView class to initialize correctly image background.

Riot/Views/RoomList/RoomCollectionViewCell.m

@@ -72,6 +72,8 @@
     self.directRoomBorderView.layer.borderWidth = 3;
     
     self.editionArrowView.backgroundColor = kRiotSecondaryBgColor;
+    
+    self.roomAvatar.defaultBackgroundColor = [UIColor clearColor];
 }
 
 - (void)layoutSubviews
@@ -165,8 +167,6 @@
             }
         }
         
-        self.roomAvatar.backgroundColor = [UIColor clearColor];
-        
         self.directRoomBorderView.hidden = !roomCellData.roomSummary.room.isDirect;
         
         self.encryptedRoomIcon.hidden = !roomCellData.roomSummary.isEncrypted;