Seprated InvalidateMeasure out from InvalidateFormattedText.

Whoever needs both the feature, calls then explicitly.
In this way TextPresenter's subscriber for SelectionStart/End doent invalidate measure,
since it should only focus on invalidate formatted text

Fixes #1448

src/Avalonia.Controls/TextBlock.cs

@@ -120,6 +120,7 @@ namespace Avalonia.Controls
                 .Subscribe(_ =>
                 {
                     InvalidateFormattedText();
+                    InvalidateMeasure();
                 });
         }
 
@@ -370,8 +371,6 @@ namespace Avalonia.Controls
                 _constraint = _formattedText.Constraint;
                 _formattedText = null;
             }
-
-            InvalidateMeasure();
         }
 
         /// <summary>
@@ -402,6 +401,7 @@ namespace Avalonia.Controls
         {
             base.OnAttachedToLogicalTree(e);
             InvalidateFormattedText();
+            InvalidateMeasure();
         }
     }
 }