Shadows have some artifacts on iOS hardware

[ianmackenzie]

There are often some missing pixels, and occasionally a large misplaced 'shard' of shadow (as if one of the triangles of the shadow volume wasn't rendered at all). Could be that these issues go away when switching to a more robust shadow implementation (#35) - needs to be tested.

[ianmackenzie]

Resolved largely by updating to use higher precision as appropriate (#74), as well as some other tricks like using a small polygon offset on shadow volumes and tightening near/far depth values.