Using different TextFormattingModes with FlowDocument can sometimes use the incorrect mode.

* .NET Core Version: any
* Windows version: Windows 10 1903
* Does the bug reproduce also in WPF for .NET Framework 4.8?: Yes

**Problem description:**

Using `TextOptions.SetTextFormattingMode()` on a `FlowDocument` or `RichTextBox` should use the specified mode when using different modes in the same application.

**Actual behavior:**

The actual mode used is somewhat random, and you cannot change it after it is set.

**Expected behavior:**

The mode specified should be used.

**Minimal repro:**

Create/use many FlowDocument (or RichTextBox) objects with different formatting modes. E.g.

```
var richText1 = new RichTextBox();
TextOptions.SetTextFormattingMode(richText1, TextFormattingMode.Ideal);
var richText2 = new RichTextBox();
TextOptions.SetTextFormattingMode(richText2, TextFormattingMode.Display);
```

I have tracked this down to a bug in [`PtsCache.AcquireContextCore`](https://github.com/dotnet/wpf/blob/8c7ab1592b6c70e732106aeabf58f215d4b71c22/src/Microsoft.DotNet.Wpf/src/PresentationFramework/MS/Internal/PtsHost/PtsCache.cs#L181), where it does not take the desired textFormattingMode into account when looking up an existing context to use from the cache.