Material overlap problem

[morenara]

Currently, the refractive index of overlapped area become addition between two refractive index.
n_total = n1 + n2 = 1.5+1.5 = 3
It somehow makes sense,
but most of the case, our intention of optical simulation is not merging (or fusing) materials,
(We are not a material scientist!)
but immersing one into the other.
For example the glass ball in the tank of water.

Could you add some options for how to treat overlapped area?

1. addition b/w materials
2. selection one refractive index based on priority (similar as layer orders in other apps)

[ricktu288]

The refractive indices multiply, not add.

Related to #37, #51, #109.