Expose some Immutable

[workgroupengineering]

if I need to create IImmutableBrush I have to do:

new ImageBrush(bitmap)
        {
            AlignmentX = AlignmentX.Left,
            AlignmentY = AlignmentY.Top,
            Stretch = Stretch.Fill
        }.ToImmutable();

I think it's a waste to create the mutable version when I already know I need an immutable one.

We could make the constructors public or better yet create a factory like Immutable.Create(...

[maxkatz6]

Most of immutable brushes are already public. Immutable image brush though (and possible some other tile brush variants) were added recently with composition renderer and were planned to be more deeply integrated at some point into rendering pipeline.