BrushToGray(id)
id to gray.
If the brush is big, this can take some time.
Note that if id specifies a palette brush, BrushToGray() will
just convert the palette colors to gray which makes this function
really fast when used with palette brushes.
Note that when using Miniwood, this function requires filters library.
BrushToGray(1)Convert brush 1 to gray.