[Grid] Fix size calculation for ranges

[kebox7]

What does the pull request do?

This PR fixes the calculation of the size of merged cells when setting the RowSpan and ColumnSpan properties.
Sample code for sandbox:

<Window xmlns="https://github.com/avaloniaui"
        xmlns:x='http://schemas.microsoft.com/winfx/2006/xaml'
        x:Class="Sandbox.MainWindow"
        Width="550"
        Height="240">

  <Grid RowDefinitions="*,Auto" RowSpacing="10"
        Grid.IsSharedSizeScope="True"
        Margin="10">

    <Grid RowDefinitions="Auto,*,Auto,Auto" RowSpacing="10"
          ColumnSpacing="10"
          ShowGridLines="True"
          Background="Moccasin">
      <Grid.ColumnDefinitions>
        <ColumnDefinition Width="Auto" />
        <ColumnDefinition />
        <ColumnDefinition Width="Auto" />
        <ColumnDefinition Width="Auto" SharedSizeGroup="C3" />
      </Grid.ColumnDefinitions>

      <ScrollViewer Grid.Row="0" Grid.RowSpan="3"
                    Grid.Column="0" Grid.ColumnSpan="3"
                    HorizontalScrollBarVisibility="Auto"
                    VerticalScrollBarVisibility="Auto">
        <TextBlock>
          0: 1234567890 1234567890 1234567890 1234567890 1234567890 1234567890<LineBreak />
          1: 1234567890 1234567890 1234567890 1234567890 1234567890 1234567890<LineBreak />
          2: 1234567890 1234567890 1234567890 1234567890 1234567890 1234567890<LineBreak />
          3: 1234567890 1234567890 1234567890 1234567890 1234567890 1234567890<LineBreak />
          4: 1234567890 1234567890 1234567890 1234567890 1234567890 1234567890<LineBreak />
          5: 1234567890 1234567890 1234567890 1234567890 1234567890 1234567890<LineBreak />
          6: 1234567890 1234567890 1234567890 1234567890 1234567890 1234567890<LineBreak />
          7: 1234567890 1234567890 1234567890 1234567890 1234567890 1234567890<LineBreak />
          8: 1234567890 1234567890 1234567890 1234567890 1234567890 1234567890<LineBreak />
          9: 1234567890 1234567890 1234567890 1234567890 1234567890 1234567890
        </TextBlock>
      </ScrollViewer>

      <Button Grid.Row="0" Grid.Column="3"
              Width="100" Height="40"
              HorizontalContentAlignment="Center"
              VerticalContentAlignment="Center"
              Content="Button R0" />
      <Button Grid.Row="2" Grid.Column="3"
              Width="100" Height="40"
              HorizontalContentAlignment="Center"
              VerticalContentAlignment="Center"
              Content="Button R2" />

      <Button Grid.Row="3" Grid.Column="0"
              Width="100" Height="40"
              HorizontalContentAlignment="Center"
              VerticalContentAlignment="Center"
              Content="Button C0" />
      <Button Grid.Row="3" Grid.Column="2"
              Width="100" Height="40"
              HorizontalContentAlignment="Center"
              VerticalContentAlignment="Center"
              Content="Button C2" />
    </Grid>

    <Grid Grid.Row="1" Background="Moccasin">
      <Grid.ColumnDefinitions>
        <ColumnDefinition />
        <ColumnDefinition Width="Auto" SharedSizeGroup="C3" />
      </Grid.ColumnDefinitions>

      <TextBlock Grid.Column="1"
                 Height="20"
                 Text="1234567890"/>
    </Grid>
  </Grid>
</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

[avaloniaui-bot]

You can test this PR using the following package version. 12.0.999-cibuild0055977-alpha. (feed url: https://nuget-feed-all.avaloniaui.net/v3/index.json) [PRBUILDID]

[avaloniaui-bot]

You can test this PR using the following package version. 12.0.999-cibuild0055985-alpha. (feed url: https://nuget-feed-all.avaloniaui.net/v3/index.json) [PRBUILDID]

[avaloniaui-bot]

You can test this PR using the following package version. 12.0.999-cibuild0055989-alpha. (feed url: https://nuget-feed-all.avaloniaui.net/v3/index.json) [PRBUILDID]

[MrJul]

I was really confused at the removal of percentReferenceSize here (we ideally don't want to touch code that isn't related to spacing). But apparently this parameter was never used! It's the same thing in the original WPF source code.

[MrJul]

LGTM, thank you!