fix centralization of buttons.

src/Avalonia.Themes.Default/ScrollBar.xaml

@@ -5,12 +5,12 @@
       <ControlTemplate>
         <Border Background="{DynamicResource ThemeControlMidBrush}">
           <Grid RowDefinitions="Auto,*,Auto">
-            <RepeatButton Name="PART_LineUpButton"
+            <RepeatButton Name="PART_LineUpButton" HorizontalAlignment="Center"
                           Classes="repeat"
                           Grid.Row="0"
                           Focusable="False"
                           MinHeight="{DynamicResource ScrollBarThickness}">
-              <Path Data="M 1,5 H 11 L 6,0 Z " />
+              <Path Data="M 0 5 L 10 5 L 5 0 Z" />
             </RepeatButton>
             <Track Grid.Row="1"
                    Grid.Column="1"
@@ -32,13 +32,13 @@
               </Track.IncreaseButton>
               <Thumb Name="thumb"/>
             </Track>
-            <RepeatButton Name="PART_LineDownButton"
+            <RepeatButton Name="PART_LineDownButton" HorizontalAlignment="Center"
                           Classes="repeat"
                           Grid.Row="2"
                           Grid.Column="2"
                           Focusable="False"
                           MinHeight="{DynamicResource ScrollBarThickness}">
-              <Path Data="M 1,0 6,5 11,0 Z" />
+              <Path Data="M 0 0 L 5 5 L 10 0 Z" />
             </RepeatButton>
           </Grid>
         </Border>