Sets the alpha value at offset <code>x</code> in
scanline <code>y</code> in the receiver's alpha data.
The alpha value must be between 0 (transparent)
and 255 (opaque).
@param x the x coordinate of the alpha value to set
@param y the y coordinate of the alpha value to set
@param alpha the value to set the alpha to
@exception IllegalArgumentException <ul>
<li>ERROR_INVALID_ARGUMENT - if x or y is out of bounds</li>
</ul>
Sets the alpha value at offset <code>x</code> in scanline <code>y</code> in the receiver's alpha data. The alpha value must be between 0 (transparent) and 255 (opaque).
@param x the x coordinate of the alpha value to set @param y the y coordinate of the alpha value to set @param alpha the value to set the alpha to
@exception IllegalArgumentException <ul> <li>ERROR_INVALID_ARGUMENT - if x or y is out of bounds</li> </ul>