Improve the aabb intersection code

Optimized the intersection code and delayed calculation we don't need
until later. This should make it much faster when working with many
bounding volumes per pick, e.g., if we introduce bounding volume
hierarchies.

Change-Id: Ib47a6de384ec257315ca96046ba8f3cf20ac0bfb
Reviewed-by: Andy Nichols <andy.nichols@qt.io>