WPF apps crash if they display text with too many combining marks

* .NET Core Version: 3.0.100-preview-010024
* Windows version: (`winver`) win 10
* Does the bug reproduce also in WPF for .NET Framework 4.8?: yes

**Problem description:**

If there are too many combining marks or other special chars on one line and you try to show this text in a WPF textbox or WPF text editor, the application crashes.

**Actual behavior:**

Crash

**Expected behavior:**

The extra special chars are either ignored or a .NET exception is thrown.

**Minimal repro:**

Paste this text in a WPF textbox or a WPF text editor (it's `new string ('\u0483', 550)`)

```
҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃҃
```

Here's code that generates it, don't try to display it in the VS C# REPL, it will crash VS:

```C#
File.WriteAllLines(@"c:\some\dir\a.txt", new[] {new string ('\u0483', 550) })
```