code: public void setRGB_preserveAlpha(int r, int g, int b){
  		setValue((getValue() & 0xff000000 ) | Pixel.argb(0, r, g, b));
  	}
docs: Sets an RGB value at the position currently referenced by this Pixel.
  The present alpha value will not be altered by this operation.
  Each channel value is assumed to be 8bit and otherwise truncated.
  @param r red
  @param g green
  @param b blue
  @throws ArrayIndexOutOfBoundsException if this Pixel's index is not in
  range of the Img's data array.
  @see #setRGB_fromDouble_preserveAlpha(double, double, double)
  @since 1.2