NullReferenceException is thrown when trying to reuse an ImageBrush<T> #881

Prerequisites

• I have written a descriptive issue title
• I have verified that I am running the latest version of ImageSharp
• I have verified if the problem exist in both DEBUG and RELEASE mode
• I have searched open and closed issues to ensure it has not already been reported

Description

A NullReferenceException is thrown when trying to reuse an ImageBrush<T>. Looks like the source image is being disposed during the first Fill(...) call.

Steps to Reproduce

using (var src = new Image<Rgba32>(5, 5))
{
    var brush = new ImageBrush<Rgba32>(src);
    using (var dest = new Image<Rgba32>(10, 10))
    {
        dest.Mutate(c => c.Fill(brush, new Rectangle(0, 0, 10, 10)));
        dest.Mutate(c => c.Fill(brush, new Rectangle(0, 0, 10, 10)));
    }
}

System Configuration

• ImageSharp version: current main
• Other ImageSharp packages and versions: n.a.
• Environment (Operating system, version and so on): Any
• .NET Framework version: Any
• Additional information: -