Consider offsets for bezels

[workergnome]

Should we have some sort of offset, that allows particles to go offscreen? This would allow bezels, and would also handle larger particles that might not want to vanish when their center point hits the edge...

[gnastacast]

My pull request for the p5 example uses an offset. Since both Y and X are 0-1 you can just do something like

float xPos = (p.pos.x-xBorder)width(1+xBorder*2);