How can a brush which is used for the BackgroundProperty of a WPF control be given a margin?

In WPF, I have a TextBox with a BackgroundProperty set to a custom brush. I want to give this brush some kind of visual buffer from the border of the TextBox. The brush is a GlyphRunBrush which acts much like an ImageBrush, but has a rasterized glyph run as the brush source.

How can I do this?

Example:

One way would be to apply a transformation to the brush itself, like so:

<TextBox>
    <TextBox.Background>
        <ImageBrush ImageSource="/MyApp;component/Search.ico"
                    AlignmentX="Right" Stretch="Uniform">
            <ImageBrush.Transform>
                <TransformGroup>
                    <TranslateTransform X="-5"/>
                </TransformGroup>
            </ImageBrush.Transform>
        </ImageBrush>
    </TextBox.Background>
</TextBox>

You could also try changing the width on the brush's Viewport property, but since you're aligning it on the right side, that may be more complicated than it needs to be.