Center Image and Rectangle with Size

[Shadowblitz16]

I have this...

<Window xmlns="https://github.com/avaloniaui"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:vm="using:SuperX.Avalonia.ViewModels"
        xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
        xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
        mc:Ignorable="d" d:DesignWidth="800" d:DesignHeight="450"
        x:Class="SuperX.Avalonia.Views.GraphicWindow"
        x:DataType="vm:GraphicWindowViewModel"
        Title="GraphicWindow">
    
    <Design.DataContext>
        <!-- This only sets the DataContext for the previewer in an IDE,
             to set the actual DataContext for runtime, set the DataContext property in code (look at App.axaml.cs) -->
        <vm:GraphicWindowViewModel/>
    </Design.DataContext>

    <Grid RowDefinitions="Auto, *">
        <Menu Grid.Row="0">
            <MenuItem Header="_File">
                <MenuItem Header="Import"/>
            </MenuItem>
        </Menu>
        <Grid Grid.Row="1" ColumnDefinitions="*, 256" RowDefinitions="*">
            <Panel Grid.Column="0" Grid.Row="0" Name="Viewport" Background="Black">16
                
                <Grid HorizontalAlignment="Center" VerticalAlignment="Center">
                    <Border BorderBrush="Cyan" BorderThickness="1"  Margin="{Binding GfxOffset}" Width="{Binding GfxWidth}" Height="{Binding GfxHeight}" />        
                    <Border BorderBrush="Yellow" BorderThickness="1"  Margin="{Binding HitOffset}" Width="{Binding HitWidth}" Height="{Binding HitHeight}" />   
                </Grid>
            </Panel>
            <Border Grid.Column="1" Grid.Row="0">
                <Grid ColumnDefinitions="*,*" RowDefinitions="Auto, Auto, Auto, Auto, Auto, Auto, Auto, Auto, Auto, Auto">
                    <Label Grid.Column="0" Grid.Row="0" VerticalAlignment="Center">Frames:</Label>
                    <NumericUpDown Grid.Column="1" Grid.Row="0" FormatString="N0" ParsingNumberStyle="Integer"  Value="1" Minimum="1" Maximum="65536"></NumericUpDown>
                    
                    <Label Grid.Column="0" Grid.Row="1" VerticalAlignment="Center">FrameSpeed:</Label>
                    <NumericUpDown Grid.Column="1" Grid.Row="1" FormatString="N0" ParsingNumberStyle="Integer"  Value="8" Minimum="1" Maximum="65536"></NumericUpDown>
                    
                    <Label Grid.Column="0" Grid.Row="2" VerticalAlignment="Center">GfxWidth:</Label>
                    <NumericUpDown Grid.Column="1" Grid.Row="2" FormatString="N0" ParsingNumberStyle="Integer"  Value="16" Minimum="1" Maximum="512"></NumericUpDown>
                    
                    <Label Grid.Column="0" Grid.Row="3" VerticalAlignment="Center">GfxHeight:</Label>
                    <NumericUpDown Grid.Column="1" Grid.Row="3" FormatString="N0" ParsingNumberStyle="Integer"  Value="16" Minimum="1" Maximum="512"></NumericUpDown>
                    
                    <Label Grid.Column="0" Grid.Row="4" VerticalAlignment="Center">GfxOffsetX:</Label>
                    <NumericUpDown Grid.Column="1" Grid.Row="4" FormatString="N0" ParsingNumberStyle="Integer"  Value="16" Minimum="-256" Maximum="255"></NumericUpDown>
                    
                    <Label Grid.Column="0" Grid.Row="5" VerticalAlignment="Center">GfxOffsetY:</Label>
                    <NumericUpDown Grid.Column="1" Grid.Row="5" FormatString="N0" ParsingNumberStyle="Integer"  Value="16" Minimum="-256" Maximum="255"></NumericUpDown>
                    
                    
                    <Label Grid.Column="0" Grid.Row="6" VerticalAlignment="Center">HitWidth:</Label>
                    <NumericUpDown Grid.Column="1" Grid.Row="6" FormatString="N0" ParsingNumberStyle="Integer"  Value="16" Minimum="1" Maximum="512"></NumericUpDown>
                    
                    <Label Grid.Column="0" Grid.Row="7" VerticalAlignment="Center">HitHeight:</Label>
                    <NumericUpDown Grid.Column="1" Grid.Row="7" FormatString="N0" ParsingNumberStyle="Integer"  Value="16" Minimum="1" Maximum="512"></NumericUpDown>
                    
                    <Label Grid.Column="0" Grid.Row="8" VerticalAlignment="Center">HitOffsetX:</Label>
                    <NumericUpDown Grid.Column="1" Grid.Row="8" FormatString="N0" ParsingNumberStyle="Integer"  Value="16" Minimum="-256" Maximum="255"></NumericUpDown>
                    
                    <Label Grid.Column="0" Grid.Row="9" VerticalAlignment="Center">HitOffsetY:</Label>
                    <NumericUpDown Grid.Column="1" Grid.Row="9" FormatString="N0" ParsingNumberStyle="Integer"  Value="16" Minimum="-256" Maximum="255"></NumericUpDown>
                </Grid>
            </Border>
        </Grid>
    </Grid>
</Window>

I am currently using a panel because I don't know how to center a item in a canvas.
But then I ran into the issue of not being able to set an offset for it in a grid.