It is a bit cleaner to do all of the necessary looping over lists of
Geoms within a straight-forward Geom.boundingBox function rather than
looping over :combine every time (or reimplementing :combine in some
cases). Geom:combine can be trivially reimplemented in terms of
Geom.boundingBox as well.
Signed-off-by: Aleksa Sarai <cyphar@cyphar.com>