Add extra margin space to flow equal to height of leaderboard

This ensures the content is always on screen, but also accounts for the
fact that scroll operations without animation were actually forcing the
local score to a location it can't usually reside at.

Basically, the local score was in the scroll extension region (due to always trying
to scroll the local player to the middle of the display, but there being
no other content below the local player to scroll up by).

osu.Game/Screens/Play/HUD/GameplayLeaderboard.cs

@@ -96,6 +96,7 @@ namespace osu.Game.Screens.Play.HUD
 
             int displayCount = Math.Min(Flow.Count, maxPanels);
             Height = displayCount * (GameplayLeaderboardScore.PANEL_HEIGHT + Flow.Spacing.Y);
+            Flow.Margin = new MarginPadding { Bottom = Height };
             requiresScroll = displayCount != Flow.Count;
 
             return drawable;