Provide API for progressive tree building

[gkjohnson]

Provide an API for progressive tree building, which can be slow for large meshes at the moment. Some ideas:

// uses requestIdleTick or requestAnimationFrame to build the tree and
// provides a promise and variable for whether or not the tree is done
const perFrameTime = 1;
geometry.computeBoundsTree(Strategy.SAH, perFrameTime).then(...);
console.log(geometry.boundsTree.ready); // false

// provides function for running the next tick of the building
geometry.computeBoundsTreeProgressively();

// every frame until finished
if (!geometry.boundsTree.ready) geometry.boundsTree.build(perFrameTime);

this would be easy to do with a generator

[gkjohnson]

Fixed by #131