Fix TextDecoration width

Fix LineBreak length

samples/Sandbox/MainWindow.axaml

@@ -1,17 +1,5 @@
 <Window xmlns="https://github.com/avaloniaui"
         xmlns:x='http://schemas.microsoft.com/winfx/2006/xaml'
         x:Class="Sandbox.MainWindow">
-  <RichTextBlock 
-    HorizontalAlignment="Center" 
-    VerticalAlignment="Center" 
-    Background="AliceBlue" 
-    Padding="5" 
-    IsTextSelectionEnabled="True" 
-    FontSize="18" 
-    FontWeight="Bold" 
-    SelectionStart="24" 
-    SelectionEnd="23">
-    <LineBreak/>
-    <Run>أَبْجَدِيَّة عَرَبِيَّة</Run>
-  </RichTextBlock>
+  <TextBlock TextDecorations="Underline" Text="Underline "/>
 </Window>

src/Avalonia.Base/Media/GlyphRun.cs

@@ -445,7 +445,7 @@ namespace Avalonia.Media
         /// </returns>
         public int FindGlyphIndex(int characterIndex)
         {
-            if (GlyphClusters == null)
+            if (GlyphClusters == null || GlyphClusters.Count == 0)
             {
                 return characterIndex;
             }

src/Avalonia.Base/Media/TextDecoration.cs

@@ -209,7 +209,7 @@ namespace Avalonia.Media
             var pen = new Pen(Stroke ?? defaultBrush, thickness,
                 new DashStyle(StrokeDashArray, StrokeDashOffset), StrokeLineCap);
 
-            drawingContext.DrawLine(pen, origin, origin + new Point(glyphRun.Size.Width, 0));
+            drawingContext.DrawLine(pen, origin, origin + new Point(glyphRun.Metrics.Width, 0));
         }
     }
 }

src/Avalonia.Controls/Documents/LineBreak.cs

@@ -1,7 +1,6 @@
 using System;
 using System.Collections.Generic;
 using System.Text;
-using Avalonia.LogicalTree;
 using Avalonia.Media.TextFormatting;
 using Avalonia.Metadata;
 
@@ -22,7 +21,7 @@ namespace Avalonia.Controls.Documents
 
         internal override void BuildTextRun(IList<TextRun> textRuns)
         {
-            textRuns.Add(new TextEndOfLine());
+            textRuns.Add(new TextEndOfLine(Environment.NewLine.Length));
         }
 
         internal override void AppendText(StringBuilder stringBuilder)