This function can be used to reduce the intensity of the alpha channel
associated with the specified brush. Every alpha pixel is multiplied
by the ratio you pass in argument 2. This ratio must be between 0 and
255. A ratio of 255 means 1.0 or 100% whereas 0 means 0.0 or 0%. Thus,
if you want to reduce the alpha transparency of all pixels by 50%, you
would have to pass 128 in the
ratio can also be a string containing a percent specification, e.g.