This adds an option to remove the bottom of the viewport as a mandatory
platform. When this is set, pieces will fall through the bottom of the
viewport instead of landing on it. This is useful when adding confetti
to pages that have a vertical scroll, when you scroll the confetti ends
up looking like its floating on the page.

This change also switches the position of the canvas that holds the
confetti from fixed to absolute, so that confetti sticks onto the
platform it landed on when the page scrolls, instead of being sticky to
the viewport when scrolling.

Saw that behavior was normal with the option disabled, and that

confetti falls through the the bottom of the viewport when the option

is set.