Always distribute specified line height evenly

[Gillibald]

What does the pull request do?

<Window xmlns="https://github.com/avaloniaui"
        xmlns:x='http://schemas.microsoft.com/winfx/2006/xaml'
        x:Class="Sandbox.MainWindow">
  <Window.Resources>
    <ControlTheme x:Key="WithLineHeightExample" TargetType="TextBlock" >
      <Setter Property="FontWeight" Value="SemiBold" />
      <Setter Property="FontSize" Value="18" />
      <Setter Property="LineHeight" Value="50" />
    </ControlTheme>

    <ControlTheme x:Key="WithoutLineHeightExample" TargetType="TextBlock">
      <Setter Property="FontWeight" Value="SemiBold" />
      <Setter Property="FontSize" Value="18" />
      <Setter Property="LineHeight" Value="NaN" />
    </ControlTheme>

    <ControlTheme x:Key="WithNegativeLineHeightExample" TargetType="TextBlock">
      <Setter Property="FontWeight" Value="SemiBold" />
      <Setter Property="FontSize" Value="18" />
      <Setter Property="LineHeight" Value="18"/>
    </ControlTheme>
  </Window.Resources>

  <Border Padding="20">
    <StackPanel Spacing="10">
      <Border BorderThickness="1" BorderBrush="Black" Padding="0" Margin="10">
        <TextBlock Theme="{StaticResource WithLineHeightExample}"
                   Text="The quick brown fox jumps over the lazy dog (LH=50)" />
      </Border>
      <Border BorderThickness="1" BorderBrush="Black" Padding="0" Margin="10">
        <TextBlock Theme="{StaticResource WithoutLineHeightExample}"
                   Text="The quick brown fox jumps over the lazy dog (LH=NaN)" />
      </Border>
      <Border BorderThickness="1" BorderBrush="Black" Padding="0" Margin="10">
        <TextBlock Theme="{StaticResource WithNegativeLineHeightExample}"
                   Text="The quick brown fox jumps over the lazy dog (LN=18)" />
      </Border>
    </StackPanel>
  </Border>
</Window>

What is the current behavior?

What is the updated/expected behavior with this PR?

How was the solution implemented (if it's not obvious)?

Checklist

• Added unit tests (if possible)?
• Added XML documentation to any related classes?
• Consider submitting a PR to https://github.com/AvaloniaUI/avalonia-docs with user documentation

Breaking changes

Obsoletions / Deprecations

Fixed issues

Fixes: #13873

[avaloniaui-bot]

You can test this PR using the following package version. 11.2.999-cibuild0047600-alpha. (feed url: https://nuget-feed-all.avaloniaui.net/v3/index.json) [PRBUILDID]

[halgari]

This is awesome! Thanks!