Make sure we don't crash when RTL text is wrapped

src/Avalonia.Visuals/Media/TextFormatting/TextFormatterImpl.cs

@@ -126,7 +126,15 @@ namespace Avalonia.Media.TextFormatting
 
             var lastCluster = glyphRun.GlyphClusters[glyphCount];
 
-            count = lastCluster - firstCluster;
+            if (glyphRun.IsLeftToRight)
+            {
+                count = lastCluster - firstCluster;
+            }
+            else
+            {
+                count = firstCluster - lastCluster;
+            }
+           
 
             return count > 0;
         }

src/Skia/Avalonia.Skia/TextShaperImpl.cs

@@ -64,7 +64,8 @@ namespace Avalonia.Skia
                     new ReadOnlySlice<double>(glyphAdvances),
                     new ReadOnlySlice<Vector>(glyphOffsets),
                     text,
-                    new ReadOnlySlice<ushort>(clusters));
+                    new ReadOnlySlice<ushort>(clusters),
+                    buffer.Direction == Direction.LeftToRight ? 0 : 1);
             }
         }