Implemented the gdScatter filter I wrote almost 10 years ago

Implemented the gdScatter filter I wrote almost 10 years ago

This filter is available as of libgd 2.1.0 which is our bare minimum for
external compilation of ext/gd.

The scatter filter works by iterating over all pixels in the image and
shifting them randomly based on two modifier (`plus` and `sub`) values:

dest_x = (int)(x + ((rand() % (plus - sub)) + sub));
dest_y = (int)(y + ((rand() % (plus - sub)) + sub));

Additionally the scatter filter also supports by only shifting pixels where
the current pixel being iterated is one or more colors, allowing the scatter
filter to only effect solid colors in part of an image.

Note, due to the nature of randomness and implementation, pixels who were
shifted ahead of iteration will be shifted once more and therefore the
bottom right of an image may contain a slight scatter effect due to this.