Add noise in the bits below the bottom of the file resolution

so that it will not look so gridded when drawing.

(The noise is taken from the other coordinate, so if two dots
were encoded at exactly the same location, they will still draw
right on top of each other.)
Fix the calculation of the color of partially transparent pixels.

Their color needs to be more intense than fully opaque pixels
so that they come out with the right brightness after multiplying
by the alpha channel.