This commit improves rendering performance by doing a better job of
checking whether an object is visible via its bounding sphere or not.
The engine maintains a bounding box for each EngineBaseObject that's
exactly large enough to fit every vertex. From this, it computes a
bounding sphere, and only draws objects if the sphere is within the view
frustum. Previously, the bounding sphere was always centered on the
EngineBaseObject's origin, even for models where the bounding box center
is significantly offset from the origin. Now, the bounding sphere is
always the tightest sphere which fits the bounding box.
AbigailBuccaneer