material's bothsides is now considered in triangle picking

- changed IPickingCollider interface, implementors need to consider
bothsides in testSubMeshCollision()
fixed a bug where Sprite3D would incorrectly capture mouse events
introduced zOffset parameter on Object3D for manual sorting offsets

- helps avoid proximity sortinf issues on transparent elements
in bounds picking fix

- Altered RaycastPicker so that it doesn't consider bounds only
collisions to be the selected pick when the ray starts within a
bounding volume
fixes a bug where mouse downs are not picked up in tablets

- onMouseDown was getting a null _collidingObject because the collision
check was done before the mouse position was updated ( there is no
hovering on tablets ), so this patch ensures that the collision check
gets a chance to do it with the right coordinates. Affects web as well,
but performance hit should be minimal