[id] = gl.ReadPixelsToBrush(x, y, width, height, id)
id. If there is already a brush that uses the identifier id, it will be
freed first. If you specify Nil in the id argument, gl.ReadPixelsToBrush() will automatically choose a vacant
identifier for this brush and return it to you.
See gl.ReadPixels for details.
Nil for auto id selectionNil as argument 5 (see above)